tools/decode_simple: Check avcodec_send_packet() for errors on flushing

This will not error but the API allows errors so we should check it
Fixes: CID1489999 Unchecked return value

Sponsored-by: Sovereign Tech Fund
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit 6df8bd64ff)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

tools/decode_simple.c

@@ -94,8 +94,9 @@ int ds_run(DecodeContext *dc)
             goto finish;
     }
 
-    avcodec_send_packet(dc->decoder, NULL);
-    ret = decode_read(dc, 1);
+    ret = avcodec_send_packet(dc->decoder, NULL);
+    if (ret >= 0)
+        ret = decode_read(dc, 1);
     if (ret < 0) {
         fprintf(stderr, "Error flushing: %d\n", ret);
         return ret;