ShellPkg: Update behavior for GetTime() errors.

ShellPkg: Update behavior for GetTime() errors.

Please find the attached patch that fixes the ShellPkg when gRT->GetTime() returns an error (eg: early UEFI platform bringup with a RTC controller that is not supported).
On some platforms, gRT->GetTime() might not return EFI_SUCCESS. The Shell must check the return status before to assume the returned time is valid.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Olivier Martin <Olivier.Martin@arm.com>
Reviewed-by: Jaben Carsey <Jaben.Carsey@intel.com>
git-svn-id: https://edk2.svn.sourceforge.net/svnroot/edk2/trunk/edk2@14337 6f19259b-4bc3-4df7-8a09-765794883524