deinterlace: guard against finding no suitable pattern

The code handles a -1 pattern index, and it seems plausible
that a pattern might be found later, so it seems best to not
send an element error here.

Coverity 1139766

gst/deinterlace/gstdeinterlace.c

@@ -1425,6 +1425,11 @@ gst_deinterlace_get_pattern_lock (GstDeinterlace * self, gboolean * flush_one)
     }
   }
 
+  if (pattern < 0) {
+    GST_WARNING_OBJECT (self, "Failed to select a pattern");
+    return;
+  }
+
   GST_DEBUG_OBJECT (self,
       "Final pattern match result: pa %d, ph %d, l %d, s %d", pattern, phase,
       telecine_patterns[pattern].length, score);