Fix qmlscene segfault on startup on Mac OS 10.6.

Fix qmlscene segfault on startup on Mac OS 10.6.

Task-number: QTBUG-23982

The platform plugin loader was searching for plugins
in qtbase/bin and would segfault when checking if
the qmlscene binary was a plugin.

Test if "../Plugins" actually exists before setting
it as the platform plugin path.

Change-Id: Ibd887b5c5045d23e3eb62867dc7598e09d3918ad
Reviewed-by: Bradley T. Hughes <bradley.hughes@nokia.com>
Reviewed-by: Sarah Jane Smith <sarah.j.smith@nokia.com>
Reviewed-by: Gunnar Sletta <gunnar.sletta@nokia.com>