Found by UBSan:
qmake/library/qmakeparser.cpp:278:33: runtime error: null pointer passed as argument 2, which is declared to never be null
Guard the call.
Change-Id: I99341ab439a511f366dae9344ddcc8727c33b9b6
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@theqtcompany.com>