bmv: check for len being valid in bmv_decode_frame().

It can be 0 or -1 for invalid files, which may result in invalid memory
access.

Reported-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
CC: libav-stable@libav.org

libavcodec/bmv.c

@@ -135,7 +135,7 @@ static int decode_bmv_frame(const uint8_t *source, int src_len, uint8_t *frame,
         mode += 1 + advance_mode;
         if (mode >= 4)
             mode -= 3;
-        if (FFABS(dst_end - dst) < len)
+        if (len <= 0 || FFABS(dst_end - dst) < len)
             return AVERROR_INVALIDDATA;
         switch (mode) {
         case 1: