Bug #46917: mysqd-nt installs wrong

When parsing the service installation parameter in 
default_service_handling() make sure the value of the
optional parameter doesn't overwrite it's name.

sql/mysqld.cc

@@ -4022,7 +4022,7 @@ default_service_handling(char **argv,
     if (opt_delim= strchr(extra_opt, '='))
     {
       size_t length= ++opt_delim - extra_opt;
-      strnmov(pos, extra_opt, length);
+      pos= strnmov(pos, extra_opt, length);
     }
     else
       opt_delim= extra_opt;