grilo: Fix crash due to lingering signal handler

If the grilo rhythmbox plugin is deactivated and reactivated, the
signal handler for "source-added" and "source-removed" will remain
alive causing a crash as the previous RBGriloPlugin instance does not
exist anymore.

Backtrace:
 #0  0x00007f0f9af231c8 in __GI_raise (sig=sig@entry=6) at ../sysdeps/unix/sysv/linux/raise.c:54
 #1  0x00007f0f9af2464a in __GI_abort () at abort.c:89
 #2  0x00007f0f9b7386a5 in g_assertion_message (domain=domain@entry=0x7f0f9b759fae "GLib", file=file@entry=0x7f0f9b75ec90 "/build/glib2.0-fJSoGg/glib2.0-2.48.1/./glib/ghash.c", line=line@entry=373,
     func=func@entry=0x7f0f9b75eed0 <__func__.10230> "g_hash_table_lookup_node", message=message@entry=0x3113c30 "assertion failed: (hash_table->ref_count > 0)")
     at ././glib/gtestutils.c:2429
 #3  0x00007f0f9b73873a in g_assertion_message_expr (domain=domain@entry=0x7f0f9b759fae "GLib", file=file@entry=0x7f0f9b75ec90 "/build/glib2.0-fJSoGg/glib2.0-2.48.1/./glib/ghash.c",
     line=line@entry=373, func=func@entry=0x7f0f9b75eed0 <__func__.10230> "g_hash_table_lookup_node", expr=expr@entry=0x7f0f9b75ebd8 "hash_table->ref_count > 0") at ././glib/gtestutils.c:2452
 #4  0x00007f0f9b701c4e in g_hash_table_lookup (hash_return=<synthetic pointer>, key=0x19ec730, hash_table=0x7f0f9ea9ca70) at ././glib/ghash.c:373
 #5  0x00007f0f9b701c4e in g_hash_table_lookup (hash_table=0x7f0f9ea9ca70, key=key@entry=0x19ec730) at ././glib/ghash.c:1147
 #6

  0x00007f0f74024c93 in grilo_source_removed_cb (registry=<optimized out>, grilo_source=0x19ec730 [GrlBookmarksSource], plugin=0x277d370) at rb-grilo-plugin.c:161

Reported-by: vrishab <vrishab.in@gmail.com>

https://bugzilla.gnome.org/show_bug.cgi?id=770044