MdeModulePkg/PerfLib: Add NULL pointer check for "Token"

MdeModulePkg/PerfLib: Add NULL pointer check for "Token"

"Token" is passed through the perf entry, it's may be NULL.
So we need to add NULL pointer check before reference it.

Cc: Liming Gao <liming.gao@intel.com>
Cc: Star Zeng <star.zeng@intel.com>
Cc: Jiewen Yao <jiewen.yao@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Dandan Bi <dandan.bi@intel.com>
Reviewed-by: Liming Gao <liming.gao@intel.com>