h265parse: Fix assertion when encountering an invalid NALU

Error out properly instead.

https://bugzilla.gnome.org/show_bug.cgi?id=775130
diff --git a/gst/videoparsers/gsth265parse.c b/gst/videoparsers/gsth265parse.c
index 84be119..f965a4b 100644
--- a/gst/videoparsers/gsth265parse.c
+++ b/gst/videoparsers/gsth265parse.c
@@ -883,8 +883,10 @@
         *skipsize = size - 3;
         goto skip;
       default:
-        g_assert_not_reached ();
-        break;
+        /* should not really occur either */
+        GST_ELEMENT_ERROR (h265parse, STREAM, FORMAT,
+            ("Error parsing H.265 stream"), ("Invalid H.265 stream"));
+        goto invalid_stream;
     }
   }