souphttpsrc: Reset retry_count to 0 when GST_FLOW_FLUSHING

If a lot of seek method is called very quickly, sometimes data reading
and do_request occurs while seek flush event is occurring and error
occurs because retry_count
reaches to the max. Thus, reset retry_count if flush occurs after
do_request and read_buffer.

https://bugzilla.gnome.org/show_bug.cgi?id=790199

ext/soup/gstsouphttpsrc.c

@@ -1835,6 +1835,13 @@ done:
       goto retry;
     }
   }
+
+  if (ret == GST_FLOW_FLUSHING) {
+    g_mutex_lock (&src->mutex);
+    src->retry_count = 0;
+    g_mutex_unlock (&src->mutex);
+  }
+
   return ret;
 }