Platform/ARM: fix uninitialized variable FileSize in RunAxf

Clang 14 detects a potentially uninitialized variable FileSize:

RunAxf.c:216:11: error: variable 'FileSize' is used uninitialized
                        whenever 'if' condition is false
RunAxf.c:281:38: note: uninitialized use occurs here
WriteBackDataCacheRange (FileData, FileSize);
                                    ^~~~~~~~

Reading the code it doesn't look like this can actually happen, but we
can keep clang happy by initialising FileSize to 0.

Upstream-Status: Pending
Signed-off-by: Ross Burton <ross.burton@arm.com>

diff --git a/Platform/ARM/Library/ArmShellCmdRunAxf/RunAxf.c b/Platform/ARM/Library/ArmShellCmdRunAxf/RunAxf.c
index d23739ad38..fba5e0ba30 100644
--- a/Platform/ARM/Library/ArmShellCmdRunAxf/RunAxf.c
+++ b/Platform/ARM/Library/ArmShellCmdRunAxf/RunAxf.c
@@ -136,1 +136,1 @@ ShellDynCmdRunAxfHandler (
-  UINTN                       FileSize;
+  UINTN                       FileSize = 0;