Check if mmp's encoded subject is null before used

From AOSP:
  https://android-review.googlesource.com/#/c/57709/1

Change-Id: I1de3c83b2b68128f725f8bbaf3d6764b2bdf836b
Signed-off-by: Roger Chen <cxr514033970@gmail.com>
Merge branch 'cm-10.1' into martincz
remove unused parameter

Change-Id: Ice8d62c7b4b8ef565e134a96e78d642d9c6c9141