Fix clang C++11 build

clang with "-std=c++11" does not allow implicit conversion from false
to null pointer.
Although clang with C++03 and gcc with C++03/11 treat it as warning,
not error, the conversion was in fact invalid.

Change-Id: I92629f9e313d0a00c4d39ce2e539ed140707ad5c
Reviewed-by: Jędrzej Nowacki <jedrzej.nowacki@theqtcompany.com>