avcodec/truemotion2: Fix passing null pointer to memset()

Fixes part of: 1888/clusterfuzz-testcase-minimized-5237704826552320

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 c901627918)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/truemotion2.c

@@ -915,7 +915,8 @@ static int decode_frame(AVCodecContext *avctx,
                             buf_size - offset);
         if (t < 0) {
             int j = tm2_stream_order[i];
-            memset(l->tokens[j], 0, sizeof(**l->tokens) * l->tok_lens[j]);
+            if (l->tok_lens[j])
+                memset(l->tokens[j], 0, sizeof(**l->tokens) * l->tok_lens[j]);
             return t;
         }
         offset += t;