avcodec/svq1dec: Check input space for minimum

We reject inputs that are significantly smaller than the smallest frame.
This check raises the minimum input needed before time consuming computations are performed
it thus improves the computation per input byte and reduces the potential DoS impact

Fixes: Timeout
Fixes: 472769364/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_SVQ1_DEC_fuzzer-5519737145851904

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 d538a71ad5)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/svq1dec.c

@@ -680,6 +680,11 @@ static int svq1_decode_frame(AVCodecContext *avctx, void *data,
         avctx->skip_frame >= AVDISCARD_ALL)
         return buf_size;
 
+    // Reject obviously too-small packets early: require at least one remaining bit per aligned luma macroblock.
+    // FFALIGN(s->width,  16) * FFALIGN(s->height, 16) / 256 represent the number of Macroblocks
+    if (get_bits_left(&s->gb) < FFALIGN(s->width,  16) * FFALIGN(s->height, 16) / 256)
+        return AVERROR_INVALIDDATA;
+
     result = ff_get_buffer(avctx, cur, s->nonref ? 0 : AV_GET_BUFFER_FLAG_REF);
     if (result < 0)
         return result;