ffplay: Don't assume ic->pb is non-null

This fixes crashes on exit when plaing some RealRTSP streams
(among other), e.g.
rtsp://dl.lib.brown.edu:554/areserves/1093545294660883.mp3.
These crashes have been present since 3e68b3ba.
Signed-off-by: Ronald S. Bultje <rsbultje@gmail.com>

ffplay.c

@@ -2568,9 +2568,9 @@ static int decode_thread(void *arg)
         }
         ret = av_read_frame(ic, pkt);
         if (ret < 0) {
-            if (ret == AVERROR_EOF || ic->pb->eof_reached)
+            if (ret == AVERROR_EOF || (ic->pb && ic->pb->eof_reached))
                 eof=1;
-            if (ic->pb->error)
+            if (ic->pb && ic->pb->error)
                 break;
             SDL_Delay(100); /* wait for user event */
             continue;