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/REPL.cpp

@@ -283,6 +283,8 @@ void REPL::IOHandlerInputComplete(IOHandler &io_handler, std::string &code) {
 
       PersistentExpressionState *persistent_state =
           m_target.GetPersistentExpressionStateForLanguage(GetLanguage());
+      if (!persistent_state)
+        return;
 
       const size_t var_count_before = persistent_state->GetSize();