dfa: Put our pointer check back.

The reimplementation by Libav does not prevent out of array
writes, even though it looks like it does at a quick glance.

No FFmpeg releases are affected by this

See: d1c95d2c
     3623589e
     740ebe46

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

libavcodec/dfa.c

@@ -254,6 +254,9 @@ static int decode_wdlt(GetByteContext *gb, uint8_t *frame, int width, int height
             y        += skip_lines;
             segments = bytestream2_get_le16(gb);
         }
+
+        if (frame_end <= frame)
+            return AVERROR_INVALIDDATA;
         if (segments & 0x8000) {
             frame[width - 1] = segments & 0xFF;
             segments = bytestream2_get_le16(gb);