daemon: parted: Don't crash if part-get-name called on non-partitioned device (RHBZ#1142158).

If do_part_get_parttype returns NULL, then return from the function
instead of dereferencing the NULL pointer and crashing.  Note that
do_part_get_parttype has already called reply_with_error in this case.

This fixes commit 0316d247706aab7ce7c2225472feeb51fa8b1664.

Found by: Lingfei Kong
daemon: parted: Fix error message when part-get-name is called on non-GPT (RHBZ#1142416).

The error message was bogus, fix it.

This fixes commit 0316d247706aab7ce7c2225472feeb51fa8b1664.

Found by: Lingfei Kong