[rcore] Use snprintf to prevent buffer overflow in path construction

[MykBamberg]

In some cases, specific user configurations may allow for file/directory paths that exceed the set path limit. In such cases the current implementation of ScanDirectoryFiles would cause a buffer overflow.

This change makes sure that the path string does not exceed the buffer size, failing gracefully with the following warning.

WARNING: FILEIO: Path longer than 4096 characters ([very long path]...)
WARNING: FILEIO: Read files count do not match capacity allocated

[raysan5]

@MykBamberg Thanks for the improvement. Note that I'm reviewing the implementation, I prefer to avoid the sizeof() approach for sizes.