Fix GH #17278

Fix GH #17278

This was an assertion failure in regexec.c under rare circumstances.  A
reduction of the fuzzed test case is now in pat_advanced.t

The root cause of this was that the pattern being compiled was encoded in
UTF-8 and 'use locale' was in effect, equivalent to the /l charset, and
then the charset was reset inside the pattern, to /d.  But /d in a UTF-8
patterns is illegal, hence the later assertion failure.

The solution is to reset instead to /u when the pattern is UTF-8.