crash after TxSearch destroyed
```
Thread 3 "openmonero" received signal SIGSEGV, Segmentation fault.
[Switching to Thread 0x7ffe7bee3700 (LWP 28256)]
0x0000000000a9e13e in xmreg::TxSearch::still_searching() ()
(gdb) where
#0  0x0000000000a9e13e in xmreg::TxSearch::still_searching() ()
#1  0x0000000000a59d46 in xmreg::CurrentBlockchainStatus::clean_search_thread_map() ()
#2  0x0000000000a5757e in xmreg::CurrentBlockchainStatus::start_monitor_blockchain_thread()::{lambda()#1}::operator()() const ()
#3  0x0000000000a5bbe4 in void std::_Bind_simple<xmreg::CurrentBlockchainStatus::start_monitor_blockchain_thread()::{lambda()#1} ()>::_M_invoke<>(std::_Index_tuple<>) ()
#4  0x0000000000a5bb3a in std::_Bind_simple<xmreg::CurrentBlockchainStatus::start_monitor_blockchain_thread()::{lambda()#1} ()>::operator()() ()
#5  0x0000000000a5baca in std::thread::_Impl<std::_Bind_simple<xmreg::CurrentBlockchainStatus::start_monitor_blockchain_thread()::{lambda()#1} ()> >::_M_run()
    ()
#6  0x00007ffff572fc80 in ?? () from /usr/lib/x86_64-linux-gnu/libstdc++.so.6
#7  0x00007ffff65376ba in start_thread (arg=0x7ffe7bee3700)
    at pthread_create.c:333
#8  0x00007ffff53b482d in clone ()
```
