zdb: zdb_ddt_leak_init() reads uninitialized memory when birth == 0

This was written by Jeff Bonick and was committed to OpenSolaris on
November 1, 2009. It appears that Jeff meant to continue the outer loop
iteration when ddp->ddp_phys_birth == 0, but put his check inside the
inner loop. This causes a pointer to uninitialized memory to be passed
to ddt_lookup() inside a VERIFY() statement whenever that condition is
true.

Reported-by: Coverity (CID 1524462)
Reviewed-by: Damian Szuberski <szuberskidamian@gmail.com>
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Richard Yao <richard.yao@alumni.stonybrook.edu>
Closes #14264