[EXT2FS] Fix an uninitialized variable usage RTC failure

Cherry-picking commit
bobranten/Ext4Fsd@3158b91

See bobranten/Ext4Fsd#83 for more information,
and debug traces.

## How to reproduce:

- Format a partition in Ext2 with Paragon Hard Disk Manager 2010 Professional.

- Compile ReactOS with MSVC, ensuring that the driver is compiled with
  `EXT2_HTREE_INDEX` defined and set to 1, and with Run-time error checks
  enabled with the `/RTC1` flag (this is the default in our source tree).

- Install ReactOS on this partition.

## What's being observed:

After reboot, some filesystem paths are accessed, and the code inside the
`#ifdef EXT2_HTREE_INDEX` block is taken (in ext2/src/dirctl.c#L777-L794).
For some reasons, the retrieved `rc` doesn't get the "expected" value and
the `goto errorout;` path is taken. After this label, `FileIndex` was
reset to `ByteOffset`, which remained uninitialized.
A bugcheck ensues because of the failed check (usage of uninitialized variable).

Author: HBelusca

drivers/filesystems/ext2/src/dirctl.c

@@ -968,7 +968,6 @@ Ext2QueryDirectory (IN PEXT2_IRP_CONTEXT IrpContext)
 errorout:
 
         ((PULONG)((PUCHAR)Buffer + fc.efc_prev))[0] = 0;
-        FileIndex = ByteOffset;
 
         if (Status == STATUS_BUFFER_OVERFLOW) {
             /* just return fc.efc_start/EntrySize bytes that we filled */