mllib: Avoid OCaml local roots corruption.

Because we used 'return' directly, the effect of the CAMLparam0()
macro was not undone, resulting in local roots corruption when the
visit callback throws an exception.

The visit_tests unit test picked this up, but the corruption was only
seen on aarch64.

Fixes commit 4a62e52111.

mllib/visit-c.c

@@ -123,7 +123,7 @@ visitor_function_wrapper (const char *dir,
      * return an error.
      */
     *args->exnp = Extract_exception (v);
-    return -1;
+    CAMLreturnT (int, -1);
   }
 
   /* No error, return normally. */