Do early return if we can't import the found decl for a member expr.
This follows the pre-existing scheme, e.g with E->getMemberDecl().
E->getFoundDecl().getDecl() can be null when a member expression does not involve lookup. It may involve a lookup in case of a using directive which refers to a member function in a base class template.
We faced this assert during the CTU analysis of google::protobuf v3.5.2. We tried hard to synthesize a minimal test example both by hand and by executing creduce on multiple files. Unfortunately, we were unable to reduce to such a minimal example, yet. Nevertheless, this fix solved the problem in protobuf.
To reproduce the error one must execute the analyzer with -Xclang -analyzer-config -Xclang experimental-enable-naive-ctu-analysis=true -Xclang -analyzer-config -Xclang ctu-dir=/path/to/ctu_dir