After https://reviews.llvm.org/D116539, when `m_gdb_client_up` in
`PlatformRemoteGDBServer` is not null, the connection to a server is
expected to exist. However,
`PlatformRemoteGDBServer::DisconnectRemote()` is not the only way to
close the connection;
`GDBRemoteCommunication::WaitForPacketNoLock()` can disconnect if the
server stops responding, and in this case `m_gdb_client_up` is not
cleared. The patch removes this assumption and checks the connection
status directly.
