config: fix memory leak in value_to_enum()

This could be triggered by running, for example:

    foot -o tweak.render-timer=invalid

For some reason LeakSanitizer was only detecting this leak when the
option value was invalid, even though it was occuring in either case.

config.c

@@ -614,6 +614,7 @@ value_to_enum(struct context *ctx, const char **value_map, int *res)
         valid_values[idx - 2] = '\0';
 
     LOG_CONTEXTUAL_ERR("not one of %s", valid_values);
+    free(valid_values);
     *res = -1;
     return false;
 }