1) log_msg.msg() will never be nullptr, unless logd sends an invalid
response, so it's the right idea to ASSERT() that this is true
instead of just checking and continuing.
2) Even though liblog.too_big_payload is tautological, there's no
reason not to test it, in case the assumptions that make it
tautological change.
3) We're not too worried about the return value of logging functions
or that liblog prevents them from being written (anyone can write
their own values to logd after all).
Test: liblog-unit-tests
Change-Id: I144cc7cf45c164ea5f04e0786ff0e298fd626f07
5f4c79f510