lavf/utils: add error check in av_read_frame()

In particular, fix crash when the input file contains no packets (e.g. an
ffmeta input).

libavformat/utils.c

@@ -1409,12 +1409,19 @@ int av_read_frame(AVFormatContext *s, AVPacket *pkt)
     AVStream *st;
 
     if (!genpts) {
-        ret = s->packet_buffer ? read_from_packet_buffer(&s->packet_buffer,
-                                                          &s->packet_buffer_end,
-                                                          pkt) :
+        while (1) {
+            ret = s->packet_buffer ?
+                read_from_packet_buffer(&s->packet_buffer, &s->packet_buffer_end, pkt) :
                 read_frame_internal(s, pkt);
+            if (ret < 0) {
+                if (ret == AVERROR(EAGAIN))
+                    continue;
+                else
+                    return ret;
+            }
             goto return_packet;
         }
+    }
 
     for (;;) {
         AVPacketList *pktl = s->packet_buffer;