ZED: Fix uninitialized value reads

Coverity complained about a couple of uninitialized value reads in ZED.

• zfs_deliver_dle() can pass an uninitialized string to zed_log_msg()
• An uninitialized sev.sigev_signo is passed to timer_create()

The former would log garbage while the latter is not a real issue, but
we might as well suppress it by initializing the field to 0 for
consistency's sake.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Richard Yao <richard.yao@alumni.stonybrook.edu>
Closes #14047