tools/target_dec_fuzzer: Fix memleak on open failure

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

tools/target_dec_fuzzer.c

@@ -180,8 +180,10 @@ int LLVMFuzzerTestOneInput(const uint8_t *data, size_t size) {
     }
 
     int res = avcodec_open2(ctx, c, NULL);
-    if (res < 0)
+    if (res < 0) {
+        av_free(ctx);
         return 0; // Failure of avcodec_open2() does not imply that a issue was found
+    }
 
     FDBCreate(&buffer);
     int got_frame;