Only call destroy on a client when it is not already being destroyed

Adds a check in the error listener on the client in the pool, to
prevent calling destroy on a client when it is already being
destroyed.
Without this check, if an error occurs during the ending of the
stream, such as a timeout, the client is never removed from
the pool and weird things happen.

lib/pool.js

@@ -30,7 +30,13 @@ var pools = {
           //via the pg object and then removing errored client from the pool
           client.on('error', function(e) {
             pool.emit('error', e, client);
+
+            // If the client is already being destroyed, the error
+            // occurred during stream ending. Do not attempt to destroy
+            // the client again.
+            if (!client._destroying) {
               pool.destroy(client);
+            }
           });
 
           // Remove connection from pool on disconnect