[ALSA] sound/synth/util_mem.c: remove pointless check

The Coverity checker spotted that if anyone would call this function
with 'prev == NULL', he would still get an Oops a few lines below.

Signed-off-by: Adrian Bunk <bunk@stusta.de>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Jaroslav Kysela <perex@suse.cz>