Fix Python frame unwinder issue caught by Valgrind

Fix Python frame unwinder issue caught by Valgrind

Valgrind shows:

 ==17026== Invalid write of size 8
 ==17026==    at 0x54AA80: pending_frame_invalidate (py-unwind.c:477)
 ==17026==    by 0x5AB934: do_my_cleanups (cleanups.c:155)
 ==17026==    by 0x5AB9AF: do_cleanups (cleanups.c:177)
 ==17026==    by 0x54B009: pyuw_sniffer (py-unwind.c:606)
 ==17026==    by 0x755DAC: frame_unwind_try_unwinder (frame-unwind.c:105)
 ==17026==    by 0x755EEE: frame_unwind_find_by_frame (frame-unwind.c:160)
 ==17026==    by 0x750FFA: compute_frame_id (frame.c:454)
 ==17026==    by 0x753BD6: get_prev_frame_if_no_cycle (frame.c:1781)
 ==17026==    by 0x754292: get_prev_frame_always_1 (frame.c:1955)
 ==17026==    by 0x7542DA: get_prev_frame_always (frame.c:1971)
 ==17026==    by 0x7547BE: get_prev_frame (frame.c:2213)
 ==17026==    by 0x7532BD: unwind_to_current_frame (frame.c:1450)
 ==17026==  Address 0xd27b570 is 16 bytes inside a block of size 32 free'd
 ==17026==    at 0x4A07577: free (in /usr/lib64/valgrind/vgpreload_memcheck-amd64-linux.so)
 ==17026==    by 0x54B276: gdb_Py_DECREF (python-internal.h:185)
 ==17026==    by 0x54B298: py_decref (py-utils.c:34)
 ==17026==    by 0x5AB934: do_my_cleanups (cleanups.c:155)
 ==17026==    by 0x5AB9AF: do_cleanups (cleanups.c:177)
 ==17026==    by 0x54B009: pyuw_sniffer (py-unwind.c:606)
 ==17026==    by 0x755DAC: frame_unwind_try_unwinder (frame-unwind.c:105)
 ==17026==    by 0x755EEE: frame_unwind_find_by_frame (frame-unwind.c:160)
 ==17026==    by 0x750FFA: compute_frame_id (frame.c:454)
 ==17026==    by 0x753BD6: get_prev_frame_if_no_cycle (frame.c:1781)
 ==17026==    by 0x754292: get_prev_frame_always_1 (frame.c:1955)
 ==17026==    by 0x7542DA: get_prev_frame_always (frame.c:1971)
 ==17026==

Simply invalidate the object before releasing it.

gdb/ChangeLog:
2015-08-13  Pedro Alves  <palves@redhat.com>

	* python/py-unwind.c (pyuw_sniffer): Install the invalidate
	cleanup after the decref cleanup, not before.