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
1 file changed