Log functions assume valid ThreadContext

[ljbade]

Currently the log functions assume there is a valid ThreadContext at https://github.com/mapbox/mapbox-gl-native/blob/master/src/mbgl/platform/log.cpp#L52

There should be a check to see if the ThreadContext is valid first so the log functions can be safely used from any thread.

Found while working on #1662

/cc @kkaefer