Fix warning generated by GCC.

These warnings seem to have been triggered by the recent change of
EFI_STATUS from INTN to UINTN.

(based on FatPkg commit c573d39c2a5627b44826d6f656452e8d097c97ff)

[jordan.l.justen@intel.com: Use script to relicense to 2-clause BSD]
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Jordan Justen <jordan.l.justen@intel.com>
Acked-by: Mark Doran <mark.doran@intel.com>
Acked-by: Laszlo Ersek <lersek@redhat.com>

FatPkg/EnhancedFatDxe/DirectoryManage.c

@@ -1441,6 +1441,8 @@ Returns:
   FAT_OFILE   *OFile;
   FAT_DIRENT  *DirEnt;
 
+  DirEnt = NULL;
+
   FileNameLen = StrLen (FileName);
   if (FileNameLen == 0) {
     return EFI_INVALID_PARAMETER;