ShellPkg/UefiShellLib: Avoid reading undefined content before string

ShellPkg/UefiShellLib: Avoid reading undefined content before string

https://bugzilla.tianocore.org/show_bug.cgi?id=566

In function InternalShellPrintWorker(), if the string in variable
'mPostReplaceFormat2' starts with character L'%', the following
expression:

*(ResumeLocation-1) == L'^' at line 2831

will read an undefined value before the starting of string
'mPostReplaceFormat2'.

This commit adds additional logic to avoid reading undefined content.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Hao Wu <hao.a.wu@intel.com>
Reviewed-by: Jaben Carsey <jaben.carsey@intel.com>