Fix for #489672 (Neil Norwitz): memory leak in test_sre.

Fix for #489672 (Neil Norwitz): memory leak in test_sre.

(At least for the repeatable test case that Tim produced.)

pattern_subx(): Add missing DECREF(filter) in both exit branches
(normal and error return).  Also fix a DECREF(args) that should
certainly be a DECREF(match) -- because it's inside if (!args) and
right after allocation of match.