ext4: check for zero length extent

Explicitly test for an extent whose length is zero, and flag that as a
corrupted extent.

This avoids a kernel BUG_ON assertion failure.

Tested: Without this patch, the file system image found in
tests/f_ext_zero_len/image.gz in the latest e2fsprogs sources causes a
kernel panic.  With this patch, an ext4 file system error is noted
instead, and the file system is marked as being corrupted.

https://bugzilla.kernel.org/show_bug.cgi?id=42859

Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
Cc: stable@kernel.org

fs/ext4/extents.c

@@ -300,6 +300,8 @@ static int ext4_valid_extent(struct inode *inode, struct ext4_extent *ext)
 	ext4_fsblk_t block = ext4_ext_pblock(ext);
 	int len = ext4_ext_get_actual_len(ext);
 
+	if (len == 0)
+		return 0;
 	return ext4_data_block_valid(EXT4_SB(inode->i_sb), block, len);
 }