avformat/rka: bps < 8 is invalid

Fixes: division by zero
Fixes: 57828/clusterfuzz-testcase-minimized-ffmpeg_dem_RKA_fuzzer-6571818338353152

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 167b4f56f1)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavformat/rka.c

@@ -72,7 +72,7 @@ static int rka_read_header(AVFormatContext *s)
     if (channels == 0)
         return AVERROR_INVALIDDATA;
     bps = par->extradata[13];
-    if (bps == 0)
+    if (bps < 8)
         return AVERROR_INVALIDDATA;
     size_offset = avio_rl32(s->pb);
     framepos = avio_tell(s->pb);