In Perl_fbm_compile(), use STRLEN instead of U32 to calculate BmPREVIOUS().

In Perl_fbm_compile(), use STRLEN instead of U32 to calculate BmPREVIOUS().

This should fix a theoretical bug on strings longer than 2**32 bytes where the
byte referenced by BmRARE() is at an offset beyond 2**32. I'm not sure how to
test this, as I think to trigger it one would need to have one of

a: the second argument to index as a string literal, longer than 2**32 bytes
b: a fixed string in a regex, longer than 2**32 bytes