http: Check for negative chunk sizes

A negative chunk size is illegal and would end up used as
length for memcpy, where it would lead to memory accesses
out of bounds.

Found-by: Paul Cher <paulcher@icloud.com>

CC: libav-stable@libav.org
Signed-off-by: Martin Storsjö <martin@martin.st>

libavformat/http.c

@@ -784,8 +784,9 @@ static int http_read_stream(URLContext *h, uint8_t *buf, int size)
 
                 av_log(NULL, AV_LOG_TRACE, "Chunked encoding data size: %"PRId64"'\n",
                         s->chunksize);
-
-                if (!s->chunksize)
+                if (s->chunksize < 0)
+                    return AVERROR_INVALIDDATA;
+                else if (!s->chunksize)
                     return 0;
                 break;
             }