Prevent segfaults, makemon can return null

src/makemon.c

@@ -122,6 +122,7 @@ register int x, y, n;
 		 */
 		if (enexto(&mm, mm.x, mm.y, mtmp->data)) {
 		    mon = makemon(mtmp->data, mm.x, mm.y, NO_MM_FLAGS);
+		    if (mon) {
 			mon->mpeaceful = FALSE;
 			mon->mavenge = 0;
 			set_malign(mon);
@@ -132,6 +133,7 @@ register int x, y, n;
 		    }
 		}
 	}
+}
 
 STATIC_OVL
 void

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);