ffplay: Fix invalid array index

Found-by: Thomas Guilbert <tguilbert@google.com>
Fixes: clusterfuzz_usan-2016-08-02
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit 6cd9a8b67a)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

ffplay.c

@@ -2869,7 +2869,7 @@ static int read_thread(void *arg)
         AVStream *st = ic->streams[i];
         enum AVMediaType type = st->codec->codec_type;
         st->discard = AVDISCARD_ALL;
-        if (wanted_stream_spec[type] && st_index[type] == -1)
+        if (type >= 0 && wanted_stream_spec[type] && st_index[type] == -1)
             if (avformat_match_stream_specifier(ic, st, wanted_stream_spec[type]) > 0)
                 st_index[type] = i;
     }