Prevent segfaults, makemon can return null

src/mhitu.c

@@ -2558,6 +2558,7 @@ cloneu()
 	if (u.mh <= 1) return(struct monst *)0;
 	if (mvitals[mndx].mvflags & G_EXTINCT) return(struct monst *)0;
 	mon = makemon(youmonst.data, u.ux, u.uy, NO_MINVENT|MM_EDOG);
+	if (!mon) return NULL;
 	mon->mcloned = 1;
 	mon = christen_monst(mon, plname);
 	initedog(mon);