Fix session-lifespan memory leak when a plperl function is redefined:

Fix session-lifespan memory leak when a plperl function is redefined:
we have to tell Perl it can release its compiled copy of the function
text.  Noted by Alexey Klyukin.

Back-patch to 8.2 --- the problem exists further back, but this patch
won't work without modification, and it's probably not worth the trouble.