StandaloneMmPkg: Initialize 'WillReturn' variable

The local variable 'WillReturn' was being used without prior
initialization in some code paths.
This patch ensures that 'WillReturn' is properly initialized
to prevent undefined behavior.

Signed-off-by: Zhiguang Liu <zhiguang.liu@intel.com>

StandaloneMmPkg/Core/Mmi.c

@@ -174,6 +174,7 @@ MmiManage (
   EFI_STATUS   Status;
 
   mMmiManageCallingDepth++;
+  WillReturn   = FALSE;
   Status       = EFI_NOT_FOUND;
   ReturnStatus = Status;
   if (HandlerType == NULL) {