IntelFsp2Pkg/SplitFspBin.py: Coverity scan flags issues.

IntelFsp2Pkg/SplitFspBin.py: Coverity scan flags issues.

REF: https://bugzilla.tianocore.org/show_bug.cgi?id=1706

Issues were:
1. copy_paste_error: PeOptHdr vs PePlusOptHdr.
2. invalid_operation: Invalid operation on
null-like value "roffset".
3. invalid_operation: Invalid operation on
null-like value "rsize".

Fixed them by initializing roffset and rsize as PE32
image type and overriding them when PE32+ header found.

Test:
Compared script output before and after this patch and
the result is identical.

Cc: Maurice Ma <maurice.ma@intel.com>
Cc: Nate DeSimone <nathaniel.l.desimone@intel.com>
Cc: Star Zeng <star.zeng@intel.com>
Signed-off-by: Chasel Chiu <chasel.chiu@intel.com>
Reviewed-by: Star Zeng <star.zeng@intel.com>
Reviewed-by: Nate DeSimone <nathaniel.l.desimone@intel.com>