vcap/rtsp: emit_pps_fps loop fix

If h264_offset_len was 0 (SPS/PPS not in RTSP but eg. in the stream,
as UltraGrid RTSP server currently does), _grab() returned NULL
infinitely.

Author: MartinPulec

src/video_capture/rtsp.c

@@ -370,10 +370,10 @@ vidcap_rtsp_thread(void *arg) {
  * only next would be displayed).
  */
 static struct video_frame *emit_sps_pps(struct rtsp_state *s) {
+    s->sps_pps_emitted = 1;
     if (s->vrtsp_state.h264_offset_len == 0) {
         return NULL;
     }
-    s->sps_pps_emitted = 1;
     struct video_frame *frame = vf_alloc_desc_data(s->vrtsp_state.desc);
     memcpy(frame->tiles[0].data, s->vrtsp_state.h264_offset_buffer, s->vrtsp_state.h264_offset_len);
     frame->tiles[0].data_len = s->vrtsp_state.h264_offset_len;