Add SAFE state to indicate un-probed safe tile

puzz.c

@@ -106,8 +106,18 @@ status_t check(puzz_t puzz)
 {
 	for (int y = 0; y < HEIGHT; ++y) {
 		for (int x = 0; x < WIDTH; ++x) {
-			if (puzz[x][y] != UNKNOWN && puzz[x][y] != soln[x][y])
-				return INCORRECT;
+			switch (puzz[x][y]) {
+			case UNKNOWN:
+				continue;
+			case SAFE:
+				if (soln[x][y] == MINE)
+					return INCORRECT;
+				break;
+			default:
+				if (puzz[x][y] != soln[x][y])
+					return INCORRECT;
+				break;
+			}
 		}
 	}
 	return OK;