IntelFsp2Pkg/GenCfgOpt.py: Coverity scan flags issues.

IntelFsp2Pkg/GenCfgOpt.py: Coverity scan flags issues.

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

Issue was: invalid_operation: Invalid operation on
null-like value "Base".
Fixed it by initializing Base to 0 before entering
while loop.

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>