Fix potential SEGV (as seen on CI)

29050==ERROR: AddressSanitizer: SEGV on unknown address 0x000000000020

(pc 0x7f26e12e0cf5 sp 0x7f269f092520 bp 0x7f269f092530 T19)

#0 0x7f26e12e0cf4 in

KDevelop::TopDUContext::parsingEnvironmentFile() const
/home/jenkins/builds/kdevplatform/stable-kf5-qt5/language/duchain/topducontext.cpp:550

#1 0x7f26ad42110c in parsingEnvironmentFile

/home/jenkins/builds/kdevelop/stable-kf5-qt5/languages/clang/clangparsejob.cpp:151

#2 0x7f26ad424990 in

ClangParseJob::run(QSharedPointer<ThreadWeaver::JobInterface>,
ThreadWeaver::Thread*)
/home/jenkins/builds/kdevelop/stable-kf5-qt5/languages/clang/clangparsejob.cpp:321

#3 0x7f26d3af5a6a in

ThreadWeaver::IdDecorator::run(QSharedPointer<ThreadWeaver::JobInterface>,
ThreadWeaver::Thread*)
/home/jenkins/builds/threadweaver/stable-kf5-qt5/src/iddecorator.cpp:69