gpt: "read-only" operation could clobber the pMBR in another way

gpt: "read-only" operation could clobber the pMBR in another way

A read-only operation like "parted $dev print" would overwrite $dev's
pMBR when exactly one of the primary and backup tables was corrupt.
* libparted/labels/gpt.c (gpt_read): Clear "write_back" in those
two cases.  Hans De Goede spotted this bug by inspection.
* NEWS (Bug fixes): Mention it.
* tests/t0206-gpt-print-with-corrupt-primary-clobbers-pmbr.sh: New test.
* tests/Makefile.am (TESTS): Add
t0206-gpt-print-with-corrupt-primary-clobbers-pmbr.sh.