[LLDB][ClangExpression] Prevent nullptr namespace map access during l…

…ogging

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.

Differential Revision: https://reviews.llvm.org/D130561

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

@@ -191,12 +191,12 @@ TagDecl *ClangASTSource::FindCompleteType(const TagDecl *decl) {
     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());