fix: connection leak if wait queue member cancelled

A connection could potentially be leaked if a wait queue member was
cancelled (due to timeout) before execution. In these cases we
should return the connection back to the list of connections for
future use or pruning.

NODE-2865

Author: mbroadst

lib/cmap/connection_pool.js

@@ -477,18 +477,14 @@ function processWaitQueue(pool) {
   if (pool.waitQueueSize && (maxPoolSize <= 0 || pool.totalConnectionCount < maxPoolSize)) {
     createConnection(pool, (err, connection) => {
       const waitQueueMember = pool[kWaitQueue].shift();
-      if (waitQueueMember == null) {
+      if (waitQueueMember == null || waitQueueMember[kCancelled]) {
         if (err == null) {
           pool[kConnections].push(connection);
         }
 
         return;
       }
 
-      if (waitQueueMember[kCancelled]) {
-        return;
-      }
-
       if (err) {
         pool.emit('connectionCheckOutFailed', new ConnectionCheckOutFailedEvent(pool, err));
       } else {