This is an old bug, where a plugin can get called while we're shutting
down (and have freed plugins), but it's triggered more reliably by the
new warning notification hook.
For good measure, we also make freeing a plugin self-delete.
Valgrind error file: valgrind-errors.16763
==16886== Invalid read of size 8
==16886== at 0x422919: plugins_notify (plugin.c:1096)
==16886== by 0x413919: notify_warning (notification.c:61)
==16886== by 0x412BDE: logv (log.c:251)
==16886== by 0x412A98: log_ (log.c:311)
==16886== by 0x4044BE: bcli_finished (bitcoind.c:178)
==16886== by 0x459480: destroy_conn (poll.c:244)
==16886== by 0x459499: destroy_conn_close_fd (poll.c:250)
==16886== by 0x4619E1: notify (tal.c:235)
==16886== by 0x461A7E: del_tree (tal.c:397)
==16886== by 0x461AB5: del_tree (tal.c:407)
==16886== by 0x461AB5: del_tree (tal.c:407)
==16886== by 0x461AB5: del_tree (tal.c:407)
==16886== Address 0x634a578 is 40 bytes inside a block of size 352 free'd
==16886== at 0x4C2EDEB: free (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==16886== by 0x461AFD: del_tree (tal.c:416)
==16886== by 0x461FB7: tal_free (tal.c:481)
==16886== by 0x411E0A: main (lightningd.c:841)
==16886== Block was alloc'd at
==16886== at 0x4C2DB8F: malloc (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==16886== by 0x4617CE: allocate (tal.c:245)
==16886== by 0x461E4C: tal_alloc_ (tal.c:423)
==16886== by 0x42255E: plugins_new (plugin.c:106)
==16886== by 0x41133D: new_lightningd (lightningd.c:218)
==16886== by 0x411AD4: main (lightningd.c:649)
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>