A correct fix for Issue 404: Read past end of string parsing fflags

A correct fix for Issue 404: Read past end of string parsing fflags

The previous fix actually broke the fflag parsing.  We
cannot use strcmp() here because we're comparing a null-terminated
string to a part of another string.

This fix explicitly tracks the various string lengths and
checks that they match before calling memcmp() or wmemcmp().
That avoids any buffer overrun without breaking the parser.