Resolve an issue with script command strnpcinfo

* Resolved the NPC map name lookup for floating NPC returning arbitrary values.

src/map/script.c

@@ -8501,7 +8501,8 @@ BUILDIN_FUNC(strnpcinfo)
 			name = aStrdup(nd->exname);
 			break;
 		case 4: // map name
-			name = aStrdup(map[nd->bl.m].name);
+			if (nd->bl.m >= 0)
+				name = aStrdup(map[nd->bl.m].name);
 			break;
 	}