MdeModulePkg/UdfDxe: suppress incorrect compiler warning in ReadFile()

When building the driver for DEBUG/RELEASE, GCC48/GCC49 warn about
ReadFile() possibly using "BytesLeft" without initializing it first.

This is not the case. The reads of "BytesLeft" are only reachable if
(ReadFileInfo->Flags == READ_FILE_SEEK_AND_READ). But, in that case, we
also set "BytesLeft" to "ReadFileInfo->FileDataSize", near the top of the
function.

Assign "BytesLeft" zero at the top, and add a comment that conforms to the
pending Coding Style Spec feature request at
<https://bugzilla.tianocore.org/show_bug.cgi?id=607>.

This issue was reported by Ard's and Gerd's CI systems independently.

Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Cc: Eric Dong <eric.dong@intel.com>
Cc: Paulo Alcantara <pcacjr@zytor.com>
Cc: Ruiyu Ni <ruiyu.ni@intel.com>
Cc: Star Zeng <star.zeng@intel.com>
Reported-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reported-by: Gerd Hoffmann <kraxel@redhat.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Star Zeng <star.zeng@intel.com>
Reviewed-by: Paulo Alcantara <pcacjr@zytor.com>

MdeModulePkg/Universal/Disk/UdfDxe/FileSystemOperations.c

@@ -893,6 +893,11 @@ ReadFile (
   LogicalBlockSize  = LV_BLOCK_SIZE (Volume, UDF_DEFAULT_LV_NUM);
   DoFreeAed         = FALSE;
 
+  //
+  // set BytesLeft to suppress incorrect compiler/analyzer warnings
+  //
+  BytesLeft = 0;
+
   switch (ReadFileInfo->Flags) {
   case READ_FILE_GET_FILESIZE:
   case READ_FILE_ALLOCATE_AND_READ: