s3:libads: avoid changing ADS->server.workgroup

s3:libads: avoid changing ADS->server.workgroup

ads_find_dc() uses c_domain = ads->server.workgroup and
don't expect it to get out of scope deep in resolve_and_ping_dns().

The result are corrupted domain values in the debug output.

Valgrind shows this:

 Invalid read of size 1
    at 0x483EF46: strlen (in /usr/lib/x86_64-linux-gnu/valgrind/vgpreload_memcheck-amd64-linux.so)
    by 0x608BE94: __vfprintf_internal (vfprintf-internal.c:1688)
    by 0x609ED49: __vasprintf_internal (vasprintf.c:57)
    by 0x5D2EC0F: __dbgtext_va (debug.c:1860)
    by 0x5D2ED3F: dbgtext (debug.c:1881)
    by 0x4BFFB50: ads_find_dc (ldap.c:570)
    by 0x4C001F4: ads_connect (ldap.c:704)
    by 0x4C1DC12: ads_dc_name (namequery_dc.c:84)
  Address 0xb69f6f0 is 0 bytes inside a block of size 11 free'd
    at 0x483CA3F: free (in /usr/lib/x86_64-linux-gnu/valgrind/vgpreload_memcheck-amd64-linux.so)
    by 0x4BFF0AF: ads_try_connect (ldap.c:299)
    by 0x4BFF40E: cldap_ping_list (ldap.c:367)
    by 0x4BFF75F: resolve_and_ping_dns (ldap.c:468)
    by 0x4BFFA91: ads_find_dc (ldap.c:556)
    by 0x4C001F4: ads_connect (ldap.c:704)
    by 0x4C1DC12: ads_dc_name (namequery_dc.c:84)
  Block was alloc'd at
    at 0x483B7F3: malloc (in /usr/lib/x86_64-linux-gnu/valgrind/vgpreload_memcheck-amd64-linux.so)
    by 0x60B250E: strdup (strdup.c:42)
    by 0x4FF1492: smb_xstrdup (util.c:743)
    by 0x4C10E62: ads_init (ads_struct.c:148)
    by 0x4C1DB68: ads_dc_name (namequery_dc.c:73)

BUG: https://bugzilla.samba.org/show_bug.cgi?id=14981

Signed-off-by: Stefan Metzmacher <metze@samba.org>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>