${^MPEN} had been treated as a synonym of ${^MATCH} due to a missing break;

A missing break; in Perl_gv_fetchpvn_flags() meant that the variable ${^MPEN}
had been behaving as a synonym of ${^MATCH}. Adding the break makes ${^MPEN}
behave like all other unused multi-character control character variable.
Update the comments in t/op/magic.t

(This should have been part of the previous commit, but I forgot to --amend
it)