buffer: fix incorrect unlock of the buffer mutex (for deferred callba…

…cks)

TSAN reports:
  WARNING: ThreadSanitizer: unlock of an unlocked mutex (or by a wrong thread) (pid=17111)
      #0 pthread_mutex_unlock /build/gcc/src/gcc/libsanitizer/sanitizer_common/sanitizer_common_interceptors.inc:3621 (libtsan.so.0+0x00000003b71c)
      libevent#1 evbuffer_add <null> (libevent_core-2.2.so.1+0x00000000ddb6)
      ...

    Mutex M392 (0x7b0c00000f00) created at:
      #0 pthread_mutex_init /build/gcc/src/gcc/libsanitizer/tsan/tsan_interceptors.cc:1117 (libtsan.so.0+0x0000000291af)
      libevent#1 <null> <null> (libevent_pthreads-2.2.so.1+0x000000000d46)
      ...

$ addr2line -e /lib/libevent_core-2.2.so.1 0x00000000ddb6
/src/libevent/buffer.c:1815 (discriminator 1)

Introduced-in: ae2b84b ("Replace
deferred_cbs with event_callback-based implementation.")

buffer.c

@@ -522,8 +522,8 @@ evbuffer_invoke_callbacks_(struct evbuffer *buffer)
 			evbuffer_incref_and_lock_(buffer);
 			if (buffer->parent)
 				bufferevent_incref_(buffer->parent);
+			EVBUFFER_UNLOCK(buffer);
 		}
-		EVBUFFER_UNLOCK(buffer);
 	}
 
 	evbuffer_run_callbacks(buffer, 0);