Fix oss_init() error path.

A bug similar to the one fixed in the previous patch for the alsa
writer is also present in the oss writer: If oss_init() fails the
->private_data pointer is non-NULL, but its contents have already
been freed. glibc detects this, aborts the process and spits out

	*** glibc detected *** ./para_write: double free or corruption (fasttop): 0x080622a0 ***

Fix this bug by initializing ->private_data only if oss_init()
succeeds.