[Clangd] [NFC] Fix dereference of null value (#159566)

This is a find from static analysis tool complaining about potential
dereference of `nullptr` for `RD`.

Author: zahiraam

clang-tools-extra/clangd/AST.cpp

@@ -147,7 +147,8 @@ std::string getQualification(ASTContext &Context,
     if (auto *TD = llvm::dyn_cast<TagDecl>(CurD)) {
       QualType T;
       if (const auto *RD = dyn_cast<CXXRecordDecl>(TD);
-          ClassTemplateDecl *CTD = RD->getDescribedClassTemplate()) {
+          ClassTemplateDecl *CTD =
+              RD ? RD->getDescribedClassTemplate() : nullptr) {
         ArrayRef<TemplateArgument> Args;
         if (const auto *SD = dyn_cast<ClassTemplateSpecializationDecl>(RD))
           Args = SD->getTemplateArgs().asArray();