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

Helps: CID1441939 Unchecked return value

Sponsored-by: Sovereign Tech Fund
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit a287f17db2)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/tiff.c

@@ -462,7 +462,8 @@ static void unpack_gray(TiffContext *s, AVFrame *p,
     GetBitContext gb;
     uint16_t *dst = (uint16_t *)(p->data[0] + lnum * p->linesize[0]);
 
-    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++) {
         dst[i] = get_bits(&gb, bpp);