ALSA: synth: emux: soundfont.c: Cleaning up memory leak

There is a risk for memory leak in when something unexpected happens
and the function returns.

This was largely found by using a static code analysis program called cppcheck.

[fixed a typo of kfree() by tiwai]

Signed-off-by: Rickard Strandqvist <rickard_strandqvist@spectrumdigital.se>
Signed-off-by: Takashi Iwai <tiwai@suse.de>