avcodec/tiff: Assert init_get_bits8() success in horizontal_fill()

Helps: CID1441167 Unchecked return value

Sponsored-by: Sovereign Tech Fund
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit 8814cedb07)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/tiff.c

@@ -427,7 +427,8 @@ static void av_always_inline horizontal_fill(TiffContext *s,
             uint8_t shift = is_dng ? 0 : 16 - bpp;
             GetBitContext gb;
 
-            init_get_bits8(&gb, src, width);
+            int ret = init_get_bits8(&gb, src, width);
+            av_assert1(ret >= 0);
             for (int i = 0; i < s->width; i++) {
                 dst16[i] = get_bits(&gb, bpp) << shift;
             }