[#326] pool: Fix panic that causes mutex deadlock

Two concurrent 'deleteClientFromMap' calls for
the same client may produce panic and deadlock.

First goroutine acquires lock, removes element
from the map, releases lock.

Second goroutine acquires lock, and throws panic
while trying to call 'close()' on empty struct.
Lock is never released and it causes deadlock for
other goroutines.

Signed-off-by: Alex Vanin <a.vanin@yadro.com>

pool/tree/pool.go

@@ -1009,8 +1009,10 @@ func (p *Pool) addClientToMap(hash uint64, cl client) {
 
 func (p *Pool) deleteClientFromMap(hash uint64) {
 	p.mutex.Lock()
-	_ = p.clientMap[hash].close()
-	delete(p.clientMap, hash)
+	if cli, ok := p.clientMap[hash]; ok {
+		_ = cli.close()
+		delete(p.clientMap, hash)
+	}
 	p.mutex.Unlock()
 }