Turn a memwipe in tor_process_handle_destroy() back to memset

It broke linking on tor-resolve.c, and it's not actually sanitizing
anything sensitive.  Fix for bug 7420; bug not on ony released Tor.

src/common/util.c

@@ -3808,7 +3808,7 @@ tor_process_handle_destroy(process_handle_t *process_handle,
     fclose(process_handle->stderr_handle);
 #endif
 
-  memwipe(process_handle, 0x0f, sizeof(process_handle_t));
+  memset(process_handle, 0x0f, sizeof(process_handle_t));
   tor_free(process_handle);
 }