libavutil: Add missing av_free() to cast5 selftest

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavutil/cast5.c

@@ -535,6 +535,7 @@ int main(int argc, char** argv)
             }
         }
     }
+    av_free(cs);
     return err;
 }
 #endif