Backport: Correct OOB check in PrepareInnerBorders() (#6064)

Correct OOB check in PrepareInnerBorders()

Co-authored-by: Stephen C. Wills <staphen@gmail.com>

Source/drlg_l4.cpp

@@ -1139,12 +1139,9 @@ static void uShape()
 			}
 			if (dung[i][j] == 1) {
 				// BUGFIX: check that i + 1 < 20 and j + 1 < 20 (fixed)
-				if (i + 1 < 20 && j + 1 < 20 &&
-				    dung[i][j + 1] == 1 && dung[i + 1][j + 1] == 0) {
-					hallok[j] = TRUE;
-				} else {
-					hallok[j] = FALSE;
-				}
+				bool hasSouthWestRoom = j + 1 < 20 && dung[i][j + 1] == 1;
+				bool hasSouthRoom = i + 1 < 20 && j + 1 < 20 && dung[i + 1][j + 1] == 1;
+				hallok[j] = hasSouthWestRoom && !hasSouthRoom ? TRUE : FALSE;
 				i = 0;
 			}
 		}
@@ -1177,12 +1174,9 @@ static void uShape()
 			}
 			if (dung[i][j] == 1) {
 				// BUGFIX: check that i + 1 < 20 and j + 1 < 20 (fixed)
-				if (i + 1 < 20 && j + 1 < 20 &&
-				    dung[i + 1][j] == 1 && dung[i + 1][j + 1] == 0) {
-					hallok[i] = TRUE;
-				} else {
-					hallok[i] = FALSE;
-				}
+				bool hasSouthEastRoom = i + 1 < 20 && dung[i + 1][j] == 1;
+				bool hasSouthRoom = i + 1 < 20 && j + 1 < 20 && dung[i + 1][j + 1] == 1;
+				hallok[i] = hasSouthEastRoom && !hasSouthRoom ? TRUE : FALSE;
 				j = 0;
 			}
 		}