qDebug: fix data race in qt_message_print

The setting of (static) messageHandler to qDefaultMessageHandler if null
was happening in multiple threads simultaneously, so it needs synchronization.

Used an atomic pointer in case qInstallMessageHandler is called from a thread,
but more importantly, initialized the static vars right away.

Improve auto test to ensure that qInstallMessageHandler(0) still sets the
default message handler.

Change-Id: I70335af38c1d28a1cdba1df8a79c6006f227422e
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>