Mms: Add null check for mmp

CM devices that set config_mms_content_disposition_support to false create an instance where pdu
headers are ignored.

See:
https://github.com/android/platform_frameworks_base/blob/master/core/res/res/values/config.xml#L804

Change-Id: I33b7c6878468208867a9e96133655cb7109f6415