[LLDB][ClangExpression] Prevent nullptr namespace map access during logging

[Michael137]

Summary

Some codepaths lead to namespace_map == nullptr when we get to
ClangASTSource::FindCompleteType. This occurred while debugging
an lldb session that had settings set target.import-std-module true.

In that case, with LLDBLog::Expressions logging enabled, we would
dereference a nullptr and crash.

This commit moves the logging until after we check for nullptr.

Testing

• Fixed the specific crash I was seeing while debugging an lldb session with import-std-module enabled.

[aprantl]

Nice!

lldb/source/Plugins/ExpressionParser/Clang/ClangASTSource.cpp

 TagDecl *ClangASTSource::FindCompleteType(const TagDecl *decl) {
   Log *log = GetLog(LLDBLog::Expressions);
 
   if (const NamespaceDecl *namespace_context =
           dyn_cast<NamespaceDecl>(decl->getDeclContext())) {
     ClangASTImporter::NamespaceMapSP namespace_map =
         m_ast_importer_sp->GetNamespaceMap(namespace_context);
 
-    LLDB_LOGV(log, "      CTD Inspecting namespace map{0} ({1} entries)",
-              namespace_map.get(), namespace_map->size());
-
     if (!namespace_map)
       return nullptr;
 
+    LLDB_LOGV(log, "      CTD Inspecting namespace map{0} ({1} entries)",
+              namespace_map.get(), namespace_map->size());
+
     for (const ClangASTImporter::NamespaceMapItem &item : *namespace_map) {
       LLDB_LOG(log, "      CTD Searching namespace {0} in module {1}",
                item.second.GetName(), item.first->GetFileSpec().GetFilename());
 
       TypeList types;
 
       ConstString name(decl->getName());