avcodec/dvdsubdec: error on bitmaps with size 0

Attemtping to decode them could lead to invalid writes with some fuzzed
samples.
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/dvdsubdec.c

@@ -111,6 +111,9 @@ static int decode_rle(uint8_t *bitmap, int linesize, int w, int h,
     if (start >= buf_size)
         return -1;
 
+    if (w <= 0 || h <= 0)
+        return -1;
+
     bit_len = (buf_size - start) * 8;
     init_get_bits(&gb, buf + start, bit_len);