config: initialize gs_free marked variable

Otherwise I get the following error (iwhile building in Jenkins):

In file included from ../include/nm-default.h:45:0,
                 from nm-config.c:27:
nm-config.c: In function 'nm_config_set_global_dns':
../include/gsystem-local-alloc.h:31:10: error: 'group_name' may be used uninitialized in this function [-Werror=maybe-uninitialized]
     func (*(Type*)v); \
          ^
nm-config.c:1483:17: note: 'group_name' was declared here
   gs_free char *group_name;
                 ^

0 files changed, 0 insertions, 0 deletions