FileSequenceFunctions : Fix error in ls() for sequences starting with #

[ivanimanishi]

The sequence overload of IECore::ls() could error with:

IndexError: basic_string::substr: __pos (which is 18446744073709551582) > this->size() (which is 4)

when scanning a directory that contained an entry shorter than the sequence's derived suffix. The length guard used
std::min( prefix.size(), suffix.size() ), which let short directory entries pass. The subsequent fileName.size() - suffix.size() is an unsigned subtraction that underflowed to a huge value, which was then passed to std::string::substr.

Replaced the guard with prefix.size() + suffix.size(). Since prefix and suffix are the non-overlapping leading/trailing parts of the name, a match must be at least as long as both combined.

Generally describe what this PR will do, and why it is needed.

• List specific new features and changes to project components

Related Issues

IECore.ls( fileSequencePath ) could raise an IndexError if the fileSequencePath started with # and the same directory had any files or directories with fewer characters than fileSequencePath.

Checklist

• I have read the contribution guidelines.
• I have updated the documentation, if applicable.
• I have tested my change(s) in the test suite, and added new test cases where necessary.
• My code follows the Cortex project's prevailing coding style and conventions.