avcodec/rv40: Test remaining space in loop of get_dimension()

Fixes infinite loop
Fixes: 178/fuzz-3-ffmpeg_VIDEO_AV_CODEC_ID_RV40_fuzzer

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/targets/ffmpeg
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit 1546d487cf)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/rv40.c

@@ -109,6 +109,8 @@ static int get_dimension(GetBitContext *gb, const int *dim)
         val = dim[get_bits1(gb) - val];
     if(!val){
         do{
+            if (get_bits_left(gb) < 8)
+                return AVERROR_INVALIDDATA;
             t = get_bits(gb, 8);
             val += t << 2;
         }while(t == 0xFF);