syslogd: Fix issues reported by gcc

• When forwaring a message, don't assume that f->f_num_addr_fds > 0.
• Avoid calling free() on a stack-allocated object.

Fixes: 4ecbee2760f7 ("syslogd: Open forwarding socket descriptors")
Fixes: f4b4a10abb26 ("syslogd: Move selector parsing into its own function")

Reviewed by: jfree
Differential Revision: https://reviews.freebsd.org/D47844