Fix potential NULL pointer dereference in lzc_ioctl()

Users are allowed to pass NULL to resultp, but we unconditionally assume
that they never do. When an external user does pass NULL to resultp, we
dereference a NULL pointer.

Clang's static analyzer complained about this.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Ryan Moeller <ryan@iXsystems.com>
Signed-off-by: Richard Yao <richard.yao@alumni.stonybrook.edu>
Closes #14008