avcodec/tiff: Check for 12bit gray fax

Fixes: Assertion failure
Fixes: 11898/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_TIFF_fuzzer-5759794191794176

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit ec28a85107)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/tiff.c

@@ -571,7 +571,7 @@ static int tiff_unpack_strip(TiffContext *s, AVFrame *p, uint8_t *dst, int strid
     if (s->compr == TIFF_CCITT_RLE ||
         s->compr == TIFF_G3        ||
         s->compr == TIFF_G4) {
-        if (is_yuv)
+        if (is_yuv || p->format == AV_PIX_FMT_GRAY12)
             return AVERROR_INVALIDDATA;
 
         return tiff_unpack_fax(s, dst, stride, src, size, width, lines);