vmnc: Check for integer overflow

Fixes null pointer dereference and potential out of array accesses.

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

libavcodec/vmnc.c

@@ -345,6 +345,10 @@ static int decode_frame(AVCodecContext *avctx, void *data, int *data_size, AVPac
         size_left = buf_size - (src - buf);
         switch(enc) {
         case MAGIC_WMVd: // cursor
+            if (w*(int64_t)h*c->bpp2 > INT_MAX/2 - 2) {
+                av_log(avctx, AV_LOG_ERROR, "dimensions too large\n");
+                return AVERROR_INVALIDDATA;
+            }
             if(size_left < 2 + w * h * c->bpp2 * 2) {
                 av_log(avctx, AV_LOG_ERROR, "Premature end of data! (need %i got %i)\n", 2 + w * h * c->bpp2 * 2, size_left);
                 return -1;