fix Perl #126182, out of memory due to infinite pattern recursion

fix Perl #126182, out of memory due to infinite pattern recursion

The way we tracked if pattern recursion was infinite did not work
properly. A pattern like

    "a"=~/(.(?2))((?<=(?=(?1)).))/

would loop forever, slowly eat up all available ram as it added
pattern recursion stack frames.

This patch changes the rules for recursion so that recursively
entering a given pattern "subroutine" twice from the same position
fails the match. This means that where previously we might have
seen fatal exception we will now simply fail. This means that

    "aaabbb"=~/a(?R)?b/

succeeds with $& equal to "aaabbb".