avcodec/escape124: Check that blocks are allocated before use

Fixes: NULL pointer dereference
Fixes: 57819/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_ESCAPE124_fuzzer-5077280228769792

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

libavcodec/escape124.c

@@ -155,7 +155,7 @@ static MacroBlock decode_macroblock(Escape124Context* s, GetBitContext* gb,
 
     // This condition can occur with invalid bitstreams and
     // *codebook_index == 2
-    if (block_index >= s->codebooks[*codebook_index].size)
+    if (block_index >= s->codebooks[*codebook_index].size || !s->codebooks[*codebook_index].blocks)
         return (MacroBlock) { { 0 } };
 
     return s->codebooks[*codebook_index].blocks[block_index];