Prevent segfaults, makemon can return null

src/makemon.c

@@ -122,13 +122,15 @@ 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);
-		    mon->mpeaceful = FALSE;
-		    mon->mavenge = 0;
-		    set_malign(mon);
-		    /* Undo the second peace_minded() check in makemon(); if the
-		     * monster turned out to be peaceful the first time we
-		     * didn't create it at all; we don't want a second check.
-		     */
+		    if (mon) {
+			mon->mpeaceful = FALSE;
+			mon->mavenge = 0;
+			set_malign(mon);
+			/* Undo the second peace_minded() check in makemon(); if the
+			 * monster turned out to be peaceful the first time we
+			 * didn't create it at all; we don't want a second check.
+			 */
+		    }
 		}
 	}
 }

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);