ShellPkg: Fix conditionally uninitialized variables

Fixes CodeQL alerts for CWE-457:
https://cwe.mitre.org/data/definitions/457.html

Cc: Erich McMillan <emcmillan@microsoft.com>
Cc: Michael D Kinney <michael.d.kinney@intel.com>
Cc: Michael Kubacki <mikuback@linux.microsoft.com>
Cc: Ray Ni <ray.ni@intel.com>
Cc: Zhichao Gao <zhichao.gao@intel.com>
Co-authored-by: Erich McMillan <emcmillan@microsoft.com>
Signed-off-by: Michael Kubacki <michael.kubacki@microsoft.com>
Reviewed-by: Zhichao Gao <zhichao.gao@intel.com>
Reviewed-by: Oliver Smith-Denny <osd@smith-denny.com>

ShellPkg/Application/Shell/Shell.c

@@ -1300,6 +1300,7 @@ DoStartupScript (
   CHAR16         *FullFileStringPath;
   UINTN          NewSize;
 
+  CalleeStatus    = EFI_SUCCESS;
   Key.UnicodeChar = CHAR_NULL;
   Key.ScanCode    = 0;