ext4: return error instead of panicking on special file inodes

[vvoland]

OpenFile and openFileViaInode panic with a nil pointer dereference when called on character device, block device, FIFO, or socket inodes. These inode types store device major/minor numbers instead of an extent tree, so inode.extents is nil and the call to inode.extents.blocks() crashes.

Add a nil guard that returns a descriptive error before accessing the extent tree.

Before this patch, the test TestOpenFileSpecialFileReturnsError failed with panic:

--- FAIL: TestOpenFileSpecialFileReturnsError (0.00s)
panic: runtime error: invalid memory address or nil pointer dereference [recovered, repanicked]
[signal SIGSEGV: segmentation violation code=0x1 addr=0x18 pc=0x57c15c]

goroutine 21 [running]:
testing.tRunner.func1.2({0x605880, 0x814950})
	/usr/lib/go-1.26/src/testing/testing.go:1974 +0x232
testing.tRunner.func1()
	/usr/lib/go-1.26/src/testing/testing.go:1977 +0x349
panic({0x605880?, 0x814950?})
	/usr/lib/go-1.26/src/runtime/panic.go:860 +0x13a
github.com/diskfs/go-diskfs/filesystem/ext4.(*FileSystem).OpenFile(0x341696991a40, {0x6464bf, 0x8}, 0x0)
	/home/pawel/src/go-diskfs/filesystem/ext4/ext4.go:1280 +0x27c
github.com/diskfs/go-diskfs/filesystem/ext4.TestOpenFileSpecialFileReturnsError(0x341696904248)
	/home/pawel/src/go-diskfs/filesystem/ext4/specialfile_test.go:24 +0x185
testing.tRunner(0x341696904248, 0x65e1b0)
	/usr/lib/go-1.26/src/testing/testing.go:2036 +0xea
created by testing.(*T).Run in goroutine 1
	/usr/lib/go-1.26/src/testing/testing.go:2101 +0x4c5
FAIL	github.com/diskfs/go-diskfs/filesystem/ext4	0.011s
FAIL