[ALSA] fix NULL pointer dereference in sound/synth/emux/soundfont.c

this is about coverity id #100.
It seems the if statement is negated, since the else branch calls
remove_info() with sflist->currsf as a parameter where it gets
dereferenced.

Signed-off-by: Eric Sesterhenn <snakebyte@gmx.de>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Adrian Bunk <bunk@stusta.de>