Fix OOB heap read in xsltExtModuleRegisterDynamic

xsltExtModuleRegisterDynamic would read a byte before the start of a
string under certain circumstances. I looks like this piece code was
supposed to strip characters from the end of the extension name, but
it didn't have any effect. Don't read beyond the beginning of the
string and actually strip unwanted characters.

Found with afl-fuzz and ASan.