BaseTools/EfiRom: Fix a segmentation fault from vsprintf()/vfprintf()

Fixes: https://bugzilla.tianocore.org/show_bug.cgi?id=534


Cc: Liming Gao <liming.gao@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Yonghong Zhu <yonghong.zhu@intel.com>
Reviewed-by: Liming Gao <liming.gao@intel.com>