Volatile-ize some plperl variables that must survive into PG_CATCH blocks.

This appears to be necessary to fix a failure seen on buildfarm member
sittella.  It shouldn't be necessary according to the letter of the C
standard, because we don't change the values of these variables within
the PG_TRY blocks; but somehow gcc 4.7.2 is dropping the ball.

Discussion: https://postgr.es/m/17555.1485179975@sss.pgh.pa.us