[ALSA] Fix seq_clientmgr dereferences before NULL check

Modules: ALSA sequencer

cptr->pool must be non-NULL there, so just the if (cptr->pool) is
superfluous. Thanks Takashi.

Signed-off-by: Eugene Teo <eugene.teo@eugeneteo.net>
Signed-off-by: Takashi Iwai <tiwai@suse.de>