Add a guard against an empty range in the EXT4 formatter (#641)

Adds a guard against an empty range in the EXT4 formatter to prevent a
possible crash:

> Swift/arm64e-apple-macos.swiftinterface:6314: Fatal error: Range
requires lowerBound <= upperBound

Author: dkovba

Sources/ContainerizationEXT4/EXT4+Formatter.swift

@@ -699,9 +699,11 @@ extension EXT4 {
                     for i in 0...usedGroupDescriptorBlocks {
                         bitmap[Int(i / 8)] |= 1 << (i % 8)
                     }
-                    for i in usedGroupDescriptorBlocks + 1...self.groupDescriptorBlocks {
-                        bitmap[Int(i / 8)] &= ~(1 << (i % 8))
-                        blocks -= 1
+                    if usedGroupDescriptorBlocks + 1 <= self.groupDescriptorBlocks {
+                        for i in usedGroupDescriptorBlocks + 1...self.groupDescriptorBlocks {
+                            bitmap[Int(i / 8)] &= ~(1 << (i % 8))
+                            blocks -= 1
+                        }
                     }
                 }
 

Tests/ContainerizationEXT4Tests/TestEXT4Format.swift

@@ -219,3 +219,14 @@ struct Ext4FormatTests: ~Copyable {
         #expect(regFile.sizeLow == 4)
     }
 }
+
+struct Ext4FormatEmptyRangeTests {
+    @Test func closeEmptyFilesystem() throws {
+        let fsPath = FilePath(
+            FileManager.default.temporaryDirectory
+                .appendingPathComponent(UUID().uuidString, isDirectory: false))
+        defer { try? FileManager.default.removeItem(at: fsPath.url) }
+        let formatter = try EXT4.Formatter(fsPath, minDiskSize: 32.kib())
+        try formatter.close()
+    }
+}