liblog: accept log messages with hdr_size greater than known headers

We don't need to be so strict about this comparison.  It's possible
that logd will extend the message that it passes to readers in the
future, and since we have a hdr_size parameter it can do so in a
backwards compatible way, as long as we loosen this restriction.

This keeps a sane upper bound that the hdr_size cannot be larger than
the log message itself.

Test: logcat, liblog-unit-tests
Change-Id: I8a6bea2a2d6e3315d998c51c1029e466ff06b45f