wallet: guard against exception in process_blocks

If an exception occurs, the thread needs to be joined, or it
will be deleted while still live, and terminate the process.

src/wallet/wallet2.cpp

@@ -720,6 +720,8 @@ void wallet2::refresh(uint64_t start_height, uint64_t & blocks_fetched, bool& re
     catch (const std::exception&)
     {
       blocks_fetched += added_blocks;
+      if (pull_thread.joinable())
+        pull_thread.join();
       if(try_count < 3)
       {
         LOG_PRINT_L1("Another try pull_blocks (try_count=" << try_count << ")...");