Fix out of range error in file extension check (#99)

This pull request fixes an issue where attempting to check the file
extension using substr could cause an out of range error if the file
name is shorter than 3 characters.
Added a condition to ensure the file name length is at least 3 before
performing the substr operation.
It will prevent potential crashes or undefined behavior when handling
short file names.

src/main.cpp

@@ -163,7 +163,7 @@ int main( int argc, char* argv[] )
   else
   {
     // whether it is a signature is determined by file extension *.eo.
-    bool isSignature = (file.substr(file.size()-3)==".eo");
+    bool isSignature = (file.size() >= 3 && file.substr(file.size()-3)==".eo");
     // include the file
     if (!s.includeFile(file, isSignature))
     {