[ALSA] Fix use after free in opl3_seq and opl3_oss

Don't read from free'd memory.  Also make use of the return
value, and don't register the device if something went wrong
creating the port.

Coverity #954, #955

Signed-off-by: Dave Jones <davej@redhat.com>
Signed-off-by: Adrian Bunk <bunk@stusta.de>