liblog: print message payload of zero length

logcat will crash if the log message payload is of zero length. Side
effect of possible log messages from LogKlog in eng and userdebug
builds, or lower level logging calls.

NB: The referenced bug cited an example of this native crash, this
    does not fix the problem cited in the bug about the
    com.android.music shutdown.

Bug: 25774695
Change-Id: I5c7a6ad8db640ba9bc8d34fab04ba7cc2a9a426a