4xmdec: fix integer overflow, null ptr dereference

 Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/4xm.c

@@ -428,7 +428,7 @@ static int decode_p_frame(FourXContext *f, const uint8_t *buf, int length)
         bytestream_size = FFMAX(length - bitstream_size - wordstream_size, 0);
     }
 
-    if (bitstream_size > length ||
+    if (bitstream_size > length || bitstream_size >= INT_MAX/8 ||
         bytestream_size > length - bitstream_size ||
         wordstream_size > length - bytestream_size - bitstream_size ||
         extra > length - bytestream_size - bitstream_size - wordstream_size) {