The read from a QHash needs to be protected too if other threads are
writing.
sync-up with qtc, no actual effect on qmake itself.
Fixes: QTCREATORBUG-21416
Change-Id: I75e5634e11b10056d6dbb6fdceef482ca2222ca1
Reviewed-by: Allan Sandfeld Jensen <allan.jensen@qt.io>
(cherry picked from qtcreator/5f79b5d2e5e33321cdcd00362f0d6d9442a73ec2)
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@qt.io>