FAPI: Fix length check in auth callback. #715

Job	Run time
scanbuild	6m 37s
test-tcti-config	14m 14s
test-coverage	5m 1s
build-test (ubuntu-20.04, gcc)	4m 46s
test-fuzz	2m 33s
test-mbedtls (ubuntu-20.04)	2m 53s
build-test (ubuntu-20.04, clang)	4m 38s
test-no-crypto-build	1m 19s
build-test (fedora-32, gcc)	4m 57s
test-mbedtls (ubuntu-22.04-mbedtls-3.1)	2m 42s
build-test (fedora-32, clang)	5m 15s
coverity-test	0s
build-test (opensuse-leap, gcc)	4m 50s
build-test (opensuse-leap, clang)	4m 45s
build-test (ubuntu-22.04, gcc)	4m 29s
build-test (ubuntu-22.04, clang)	4m 50s
build-test (alpine-3.15, gcc)	4m 8s
build-test (alpine-3.15, clang)	5m 0s
	1h 22m 57s

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