vl: fix memory leak spotted by valgrind

valgrind complains about:
==9276== 13 bytes in 1 blocks are definitely lost in loss record 1,046 of 3,673
==9276==    at 0x4C2845D: malloc (in /usr/lib64/valgrind/vgpreload_memcheck-amd64-linux.so)
==9276==    by 0x2EAFBB: malloc_and_trace (vl.c:2556)
==9276==    by 0x64C770E: g_malloc (in /usr/lib64/libglib-2.0.so.0.3600.3)
==9276==    by 0x4A28BD: addr_to_string (vnc.c:123)
==9276==    by 0x4A29AD: vnc_socket_local_addr (vnc.c:139)
==9276==    by 0x4A9AFE: vnc_display_local_addr (vnc.c:3240)
==9276==    by 0x2EF4FE: main (vl.c:4321)

Signed-off-by: Shannon Zhao <zhaoshenglong@huawei.com>
Signed-off-by: Shannon Zhao <shannon.zhao@linaro.org>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>
