Fix \p followed by a non-alpha

Fix \p followed by a non-alpha

See threads beginning at
    http://nntp.perl.org/group/perl.perl5.porters/231263
    http://nntp.perl.org/group/perl.perl5.porters/231389

Prior to this commit, these did not generate the pattern that would be
expected, and displayed apparently irrelevant warnings.  Now this is a
fatal error.

This resolves [perl #126187].  I don't think it's worth a perldelta
entry for this ticket, as the new error message is now in perldelta, and
this never worked properly anyway; it's just now we have a proper error
message.  Patches welcome if you disagree.