s3:lib: Initialize pointer to NULL

s3:lib: Initialize pointer to NULL

../../source3/lib/netapi/tests/netdisplay.c: In function ‘test_netquerydisplayinformation’:
../../source3/lib/netapi/tests/netdisplay.c:87:45: error: ‘current_name’ may be used uninitialized [-Werror=maybe-uninitialized]
   87 |                                 if (name && strcasecmp(current_name, name) == 0) {
      |                                             ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
../../source3/lib/netapi/tests/netdisplay.c:37:21: note: ‘current_name’ was declared here
   37 |         const char *current_name;
      |                     ^~~~~~~~~~~~
cc1: all warnings being treated as errors

Signed-off-by: Jennifer Sutton <jennifersutton@catalyst.net.nz>
Reviewed-by: Douglas Bagnall <douglas.bagnall@catalyst.net.nz>