In av1/av1_cx_iface.c the encoder_encode function loops over calls made
to av1_get_compressed_data(). If the show_existing_frame flag is set,
the frame has already been flushed and the av1_twopass_postencode_update
function already incremented gf group index. The loop must exit, or else
the superframe might contain multiple shown frames. This causes a
mismatch in the test_decode function in that
av1_cx_iface.c:ctrl_get_new_frame_image will return the frame at
last_show_frame_buf_idx whereas, av1_dx_iface.c:ctrl_get_new_frame_image
will return cm->frame_to_show

BUG=aomedia:669

Change-Id: I0765048be9bd04936e080db119b54c4050019bff