iscsi_initiator(4): Fix a potential NULL pointer dereference.

iscsi_initiator(4): Fix a potential NULL pointer dereference.

If sp can be NULL (as the rest of the code suggests), we can't use sdebug()
since that prints the string with sp->sid prepended (thereby dereferencing
NULL).

Instead, use debug() and add sp->sid to its string (with a check for NULL).

While here, fix some indentation issues.