Fix nil pointer dereference in events.go handling node removal

For the moment we have unsafe coding where we try to use 'host' object
before checking the success of it's 'get' operation.

When a DB node gets forcibly terminated we have a race condition
to get the following panic:

  panic: runtime error: invalid memory address or nil pointer dereference
  [signal SIGSEGV: segmentation violation code=0x1 addr=0x10 pc=0x632814]

  goroutine 1723 [running]:
  github.com/gocql/gocql.(*HostInfo).HostID(0xc0001500b8)
    %full-path%/host_source.go:253 +0x34
  github.com/gocql/gocql.(*Session).handleRemovedNode(
        0xc000150000, {0xc0134c10d8, 0xc0093d69b0, 0xa}, 0x45264d)
    %full-path%/events.go:243 +0x5f
  github.com/gocql/gocql.(*Session).handleNodeEvent(
        0xc000150000, {0xc01701a000, 0x1, 0x43e305})
    %full-path%/events.go:176 +0x28e
  created by github.com/gocql/gocql.(*eventDebouncer).flush
    %full-path%/events.go:67 +0xb5

So, fix it by checking the result first and only then use 'host' object.

AUTHORS

@@ -130,3 +130,4 @@ Ondrej Polakovič <ondrej.polakovic@kiwi.com>
 Sergei Karetnikov <sergei.karetnikov@gmail.com>
 Stefan Miklosovic <smiklosovic@apache.org>
 Adam Burk <amburk@gmail.com>
+Valerii Ponomarov <kiparis.kh@gmail.com>

events.go

@@ -240,8 +240,8 @@ func (s *Session) handleRemovedNode(ip net.IP, port int) {
 
 	// we remove all nodes but only add ones which pass the filter
 	host, ok := s.ring.getHostByIP(ip.String())
-	hostID := host.HostID()
 	if ok {
+		hostID := host.HostID()
 		s.ring.removeHost(hostID)
 
 		host.setState(NodeDown)