tal: don't defer-after-free if a destructor deletes itself.

tal: don't defer-after-free if a destructor deletes itself.

==10868==    at 0x109A96: notify (tal.c:220)
==10868==    by 0x109F7E: del_tree (tal.c:397)
==10868==    by 0x10A31A: tal_free (tal.c:481)
==10868==    by 0x10BE73: main (run-destructor.c:75)
==10868==  Address 0x4a60bd8 is 8 bytes inside a block of size 32 free'd
==10868==    at 0x483BA3F: free (in /usr/lib/x86_64-linux-gnu/valgrind/vgpreload_memcheck-amd64-linux.so)
==10868==    by 0x109D4F: del_notifier_property (tal.c:340)
==10868==    by 0x10A610: tal_del_notifier_ (tal.c:564)
==10868==    by 0x10A687: tal_del_destructor_ (tal.c:576)
==10868==    by 0x10B653: remove_own_destructor (run-destructor.c:35)
==10868==    by 0x109A67: notify (tal.c:235)
==10868==    by 0x109F7E: del_tree (tal.c:397)
==10868==    by 0x10A31A: tal_free (tal.c:481)
==10868==    by 0x10BE73: main (run-destructor.c:75)
==10868==  Block was alloc'd at
==10868==    at 0x483A7F3: malloc (in /usr/lib/x86_64-linux-gnu/valgrind/vgpreload_memcheck-amd64-linux.so)
==10868==    by 0x109AD5: allocate (tal.c:245)
==10868==    by 0x109C3E: add_notifier_property (tal.c:300)
==10868==    by 0x10A467: tal_add_destructor_ (tal.c:516)
==10868==    by 0x10BDFE: main (run-destructor.c:74)

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>