Fix use of released vgname and vgid

Fix use of released vgname and vgid

Avoid using of already released memory when duplicated MDA is found.

As get_pv_from_vg_by_id() may call lvmcache_label_scan() use the local copy
of the vgname and vgid on the stack as vginfo may dissapear and code was
then accessing garbage in memory.

i.e.  pvs  /dev/loop0
(when /dev/loop0 and /dev/loop1 has same MDA content)

Invalid read of size 1
   at 0x523C986: dm_hash_lookup (hash.c:325)
   by 0x440C8C: vginfo_from_vgname (lvmcache.c:399)
   by 0x4605C0: _create_vg_text_instance (format-text.c:1882)
   by 0x46140D: _text_create_text_instance (format-text.c:2243)
   by 0x47EB49: _vg_read (metadata.c:2887)
   by 0x47FBD8: vg_read_internal (metadata.c:3231)
   by 0x477594: get_pv_from_vg_by_id (metadata.c:344)
   by 0x45F07A: _get_pv_if_in_vg (format-text.c:1400)
   by 0x45F0B9: _populate_pv_fields (format-text.c:1414)
   by 0x45F40F: _text_pv_read (format-text.c:1493)
   by 0x480431: _pv_read (metadata.c:3500)
   by 0x4802B2: pv_read (metadata.c:3462)
 Address 0x652ab80 is 0 bytes inside a block of size 4 free'd
   at 0x4C2756E: free (vg_replace_malloc.c:366)
   by 0x442277: _free_vginfo (lvmcache.c:963)
   by 0x44235E: _drop_vginfo (lvmcache.c:992)
   by 0x442B23: _lvmcache_update_vgname (lvmcache.c:1165)
   by 0x443449: lvmcache_update_vgname_and_id (lvmcache.c:1358)
   by 0x443C07: lvmcache_add (lvmcache.c:1492)
   by 0x46588C: _text_read (text_label.c:271)
   by 0x466A65: label_read (label.c:289)
   by 0x4413FC: lvmcache_label_scan (lvmcache.c:635)
   by 0x4605AD: _create_vg_text_instance (format-text.c:1881)
   by 0x46140D: _text_create_text_instance (format-text.c:2243)
   by 0x47EB49: _vg_read (metadata.c:2887)

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