Fix segfault when the Wiz fails to reappear due to makemon failing

src/wizard.c

@@ -519,7 +519,7 @@ resurrect()
         mtmp = makemon(&mons[PM_WIZARD_OF_YENDOR], u.ux, u.uy, MM_NOWAIT);
         /* affects experience; he's not coming back from a corpse
            but is subject to repeated killing like a revived corpse */
-        mtmp->mrevived = 1;
+        if (mtmp) mtmp->mrevived = 1;
     } else {
         /* look for a migrating Wizard */
         verb = "elude";