vgname_from_mda now tries to parse for vgname even

vgname_from_mda now tries to parse for vgname even
if rlocn not defined (there is no metadata area).

In most cases it fails in validate_name(),
unfortunately there are situatuions, when
validate_name is ok and later code fails with
checksum error.

Reproducer:

# dd if=/dev/zero of=/dev/loop0

# pvcreate --metadatasize 637k /dev/loop0
  Physical volume "/dev/loop0" successfully created

# pvs /dev/loop0
  /dev/loop0: Checksum error
      PV         VG   Fmt  Attr PSize PFree
        /dev/loop0      lvm2 --   1.00M 1.00M

Signed-off-by: Milan Broz <mbroz@redhat.com>
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