Add missing nullptr checks.

GetPersistentExpressionStateForLanguage() can return a nullptr if it
cannot construct a typesystem. This patch adds missing nullptr checks
at all uses.

Inspired by rdar://problem/58317195

Differential Revision: https://reviews.llvm.org/D72413

lldb/source/Expression/UserExpression.cpp

@@ -396,8 +396,9 @@ UserExpression::Execute(DiagnosticManager &diagnostic_manager,
       diagnostic_manager, exe_ctx, options, shared_ptr_to_me, result_var);
   Target *target = exe_ctx.GetTargetPtr();
   if (options.GetResultIsInternal() && result_var && target) {
-    target->GetPersistentExpressionStateForLanguage(m_language)
-        ->RemovePersistentVariable(result_var);
+    if (auto *persistent_state =
+            target->GetPersistentExpressionStateForLanguage(m_language))
+      persistent_state->RemovePersistentVariable(result_var);
   }
   return expr_result;
 }