libnm: avoid compiler warning about uninitalized variable in nm_setting_bridge_port_remove_vlan_by_vid()

With LTO, compiler warns:

    libnm-core/nm-setting-bridge-port.c: In function nm_setting_bridge_port_remove_vlan_by_vid:
    libnm-core/nm-setting-bridge-port.c:252:6: error: v_start may be used uninitialized in this function [-Werror=maybe-uninitialized]
      252 |   if (v_start == vid_start && v_end == vid_end) {
          |      ^
    libnm-core/nm-setting-bridge-port.c:239:10: note: v_start was declared here
      239 |  guint16 v_start, v_end;
          |          ^
    libnm-core/nm-setting-bridge-port.c:252:28: error: v_end may be used uninitialized in this function [-Werror=maybe-uninitialized]
      252 |   if (v_start == vid_start && v_end == vid_end) {
          |                            ^
    libnm-core/nm-setting-bridge-port.c:239:19: note: v_end was declared here
      239 |  guint16 v_start, v_end;
          |                   ^

Avoid the (false positive) warning.