decodebin2: fix "Attempt to unlock mutex that was not locked"

Introduced in commit ee44337f, caused the decodebin
test_text_plain_streams unit test to abort.

https://bugzilla.gnome.org/show_bug.cgi?id=752651

gst/playback/gstdecodebin2.c

@@ -2867,8 +2867,9 @@ type_found (GstElement * typefind, guint probability,
   gst_object_unref (sink_pad);
   gst_object_unref (pad);
 
-exit:
   EXPOSE_UNLOCK (decode_bin);
+
+exit:
   return;
 }