examples: Add check and replace av_free() to avoid potential memory errors

Add check for the return value of av_packet_alloc() to avoid potential NULL pointer dereference.
Moreover, replace redundant av_free() with fprintf().

Fixes: 9a38184a14 ("examples/decode_audio: allocate the packet dynamically")
Signed-off-by: Jiasheng Jiang <jiashengjiangcool@gmail.com>
Reviewed-by: Nicolas George <george@nsup.org>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

doc/examples/decode_audio.c

@@ -128,6 +128,10 @@ int main(int argc, char **argv)
     outfilename = argv[2];
 
     pkt = av_packet_alloc();
+    if (!pkt) {
+        fprintf(stderr, "Could not allocate AVPacket\n");
+        exit(1); /* or proper cleanup and returning */
+    }
 
     /* find the MPEG audio decoder */
     codec = avcodec_find_decoder(AV_CODEC_ID_MP2);
@@ -161,7 +165,7 @@ int main(int argc, char **argv)
     }
     outfile = fopen(outfilename, "wb");
     if (!outfile) {
-        av_free(c);
+        fprintf(stderr, "Could not open %s\n", outfilename);
         exit(1);
     }