mextra follow up bit 2

<Someone> noticed the leftover zeromextra, this removes it.

Using memset() on a possibly failed mextra allocation was inapprorpriate,
so replace the newmextra() macro with a function.

Prevent a crash in christen_monst() if mextra was not initialized.

src/do_name.c

@@ -233,6 +233,7 @@ const char *name;
 		return mtmp;
 	}
 	if (has_name(mtmp)) free((genericptr_t)MNAME(mtmp));
+	if (!mtmp->mextra) mtmp->mextra = newmextra();
 
 	MNAME(mtmp) = (char *)alloc(lth);
 	if (lth) Strcpy(MNAME(mtmp), name);