Fix buffer overflow in u8_strlen

[tyler92]

This patch fixes a buffer overflow error in the u8_strlen function that happened with invalid UTF-8 input. I added one unit test and it failed without the fix with the following error:

=================================================================
==1789377==ERROR: AddressSanitizer: global-buffer-overflow on address 0x5586e133a5e4 at pc 0x5586e0f09b29 bp 0x7fff5e81a430 sp 0x7fff5e81a420
READ of size 4 at 0x5586e133a5e4 thread T0
    #0 0x5586e0f09b28 in valijson::utils::u8_nextchar(char const*, unsigned long*) valijson/include/valijson/utils/utf8_utils.hpp:38
    #1 0x5586e0f09eaf in valijson::utils::u8_strlen(char const*) valijson/include/valijson/utils/utf8_utils.hpp:50
    #2 0x5586e11bf014 in TestUtf8Utils_Utf8StringLength_Test::TestBody() valijson/tests/test_utf8_utils.cpp:50
    #3 0x5586e125b0af in void testing::internal::HandleSehExceptionsInMethodIfSupported<testing::Test, void>(testing::Test*, void (testing::Test::*)(), char const*) valijson/thirdparty/googletest/googletest/src/gtest.cc:2599
    #4 0x5586e124a4c0 in void testing::internal::HandleExceptionsInMethodIfSupported<testing::Test, void>(testing::Test*, void (testing::Test::*)(), char const*) valijson/thirdparty/googletest/googletest/src/gtest.cc:2635
    #5 0x5586e11f93fd in testing::Test::Run() valijson/thirdparty/googletest/googletest/src/gtest.cc:2674
    #6 0x5586e11fab16 in testing::TestInfo::Run() valijson/thirdparty/googletest/googletest/src/gtest.cc:2853
    #7 0x5586e11fbd30 in testing::TestSuite::Run() valijson/thirdparty/googletest/googletest/src/gtest.cc:3012
    #8 0x5586e12224dc in testing::internal::UnitTestImpl::RunAllTests() valijson/thirdparty/googletest/googletest/src/gtest.cc:5870
    #9 0x5586e125e471 in bool testing::internal::HandleSehExceptionsInMethodIfSupported<testing::internal::UnitTestImpl, bool>(testing::internal::UnitTestImpl*, bool (testing::internal::UnitTestImpl::*)(), char const*) valijson/thirdparty/googletest/googletest/src/gtest.cc:2599
    #10 0x5586e124cea9 in bool testing::internal::HandleExceptionsInMethodIfSupported<testing::internal::UnitTestImpl, bool>(testing::internal::UnitTestImpl*, bool (testing::internal::UnitTestImpl::*)(), char const*) valijson/thirdparty/googletest/googletest/src/gtest.cc:2635
    #11 0x5586e121ec62 in testing::UnitTest::Run() valijson/thirdparty/googletest/googletest/src/gtest.cc:5444
    #12 0x5586e128687b in RUN_ALL_TESTS() valijson/thirdparty/googletest/googletest/include/gtest/gtest.h:2293
    #13 0x5586e1286744 in main valijson/thirdparty/googletest/googletest/src/gtest_main.cc:51
    #14 0x7f47fd08dd8f in __libc_start_call_main ../sysdeps/nptl/libc_start_call_main.h:58
    #15 0x7f47fd08de3f in __libc_start_main_impl ../csu/libc-start.c:392
    #16 0x5586e0e02074 in _start (valijson/build/Desktop/Debug/test_suite+0xa1074)

0x5586e133a5e4 is located 12 bytes to the right of global variable 'offsetsFromUTF8' defined in 'valijson/include/valijson/utils/utf8_utils.hpp:17:23' (0x5586e133a5c0) of size 24
SUMMARY: AddressSanitizer: global-buffer-overflow valijson/include/valijson/utils/utf8_utils.hpp:38 in valijson::utils::u8_nextchar(char const*, unsigned long*)
Shadow bytes around the buggy address:
  0x0ab15c25f460: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x0ab15c25f470: 00 00 00 00 06 f9 f9 f9 f9 f9 f9 f9 00 00 00 03
  0x0ab15c25f480: f9 f9 f9 f9 00 00 00 00 05 f9 f9 f9 f9 f9 f9 f9
  0x0ab15c25f490: 00 00 00 00 06 f9 f9 f9 f9 f9 f9 f9 05 f9 f9 f9
  0x0ab15c25f4a0: f9 f9 f9 f9 06 f9 f9 f9 f9 f9 f9 f9 00 00 00 00
=>0x0ab15c25f4b0: 00 00 00 00 00 00 00 00 00 00 00 f9[f9]f9 f9 f9
  0x0ab15c25f4c0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x0ab15c25f4d0: 01 f9 f9 f9 f9 f9 f9 f9 00 f9 f9 f9 f9 f9 f9 f9
  0x0ab15c25f4e0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x0ab15c25f4f0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 07 f9
  0x0ab15c25f500: f9 f9 f9 f9 00 04 f9 f9 f9 f9 f9 f9 00 00 00 01
Shadow byte legend (one shadow byte represents 8 application bytes):
  Addressable:           00
  Partially addressable: 01 02 03 04 05 06 07 
  Heap left redzone:       fa
  Freed heap region:       fd
  Stack left redzone:      f1
  Stack mid redzone:       f2
  Stack right redzone:     f3
  Stack after return:      f5
  Stack use after scope:   f8
  Global redzone:          f9
  Global init order:       f6
  Poisoned by user:        f7
  Container overflow:      fc
  Array cookie:            ac
  Intra object redzone:    bb
  ASan internal:           fe
  Left alloca redzone:     ca
  Right alloca redzone:    cb
  Shadow gap:              cc
==1789377==ABORTING