Fix -fdelete-null-pointer-checks issues

Backtrace (reduced):
10 0x00007fc2221b9e9c in KDevelop::AbstractType::indexed() const
(this=0x0) at
/home/jgrulich/development/projects/kdevplatform/language/duchain/types/abstracttype.cpp:99
11 0x00007fc22212f4a6 in
KDevelop::Declaration::setAbstractType(KDevelop::TypePtr<KDevelop::AbstractType>)
(this=0x7fc1b40a42a0, type=...) at
/home/jgrulich/development/projects/kdevplatform/language/duchain/declaration.cpp:269
12 0x00007fc1f1eb988e in (anonymous
namespace)::Visitor::setDeclType<(CXCursorKind)10>(KDevelop::Declaration*,
KDevelop::AbstractType*) (decl=<optimized out>, type=<optimized out>,
this=<optimized out>) at
/home/jgrulich/development/projects/kdevelop/languages/clang/duchain/builder.cpp:786

CCBUG: 360707