gobex: Fix memory leak

gobex: Fix memory leak

This fixes the following leak introduced by the authentication patches:

16 bytes in 1 blocks are definitely lost in loss record 62 of 158
   at 0x4C291D4: calloc (in
/usr/lib64/valgrind/vgpreload_memcheck-amd64-linux.so)
   by 0x4E7FEC6: g_malloc0 (in /usr/lib64/libglib-2.0.so.0.3800.2)
   by 0x403226: prepare_auth_rsp.isra.4 (gobex.c:564)
   by 0x404DE4: handle_response (gobex.c:1015)
   by 0x405C7B: incoming_data (gobex.c:1351)
   by 0x4E7A2A5: g_main_context_dispatch (in
/usr/lib64/libglib-2.0.so.0.3800.2)
   by 0x4E7A627: ??? (in /usr/lib64/libglib-2.0.so.0.3800.2)
   by 0x4E7AA39: g_main_loop_run (in
/usr/lib64/libglib-2.0.so.0.3800.2)
   by 0x40DC34: test_auth (test-gobex.c:966)
   by 0x4E9E5E0: ??? (in /usr/lib64/libglib-2.0.so.0.3800.2)
   by 0x4E9E7A5: ??? (in /usr/lib64/libglib-2.0.so.0.3800.2)
   by 0x4E9EB1A: g_test_run_suite (in
/usr/lib64/libglib-2.0.so.0.3800.2)