4xm: check the return value of read_huffman_tables().

CC:libav-stable@libav.org

libavcodec/4xm.c

@@ -721,6 +721,10 @@ static int decode_i_frame(FourXContext *f, const uint8_t *buf, int length)
     }
 
     prestream = read_huffman_tables(f, prestream);
+    if (!prestream) {
+        av_log(f->avctx, AV_LOG_ERROR, "Error reading Huffman tables.\n");
+        return AVERROR_INVALIDDATA;
+    }
 
     init_get_bits(&f->gb, buf + 4, 8 * bitstream_size);