BaseTools/VolInfo: Fix VS2010/VS2012 build failure

BaseTools/VolInfo: Fix VS2010/VS2012 build failure

https://bugzilla.tianocore.org/show_bug.cgi?id=417

The commit makes the following refinements in VolInfo source codes to
avoid VS2010/VS2012 build failure:

1. Refines coding style for function 'CombinePath' to declare local
variables at the beginning of the function block.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Hao Wu <hao.a.wu@intel.com>
Reviewed-by: Yonghong Zhu <yonghong.zhu@intel.com>