avcodec/rka: check for size 1 filter

Such filters will not advance and be stuck in the current implementation

Fixes: Infinite loop
Fixes: 56052/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_RKA_fuzzer-5236218750435328

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 8874cfa2e1)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/rka.c

@@ -691,7 +691,7 @@ static int decode_filter(RKAContext *s, ChContext *ctx, ACoder *ac, int off, uns
     else
         split = size >> 4;
 
-    if (size <= 0)
+    if (size <= 1)
         return 0;
 
     for (int x = 0; x < size;) {