[Upstream] ipconfig: Ensure ifname is not NULL when setting IPv6 state

[Upstream] ipconfig: Ensure ifname is not NULL when setting IPv6 state

This is a quick fix for the issue where removal of an interface may
cause IPv6 to be disabled/enabled on other interfaces.

When removing an ipdevice in free_ipdevice(), it is ensured that the
ifname (which we get from connman_inet_ifname()) is not NULL,
before invoking set_ipv6_state(). If a NULL ifname is passed into
set_ipv6_state(), it will apply the change to all interfaces instead
of the intended one.

Change-Id: I2b0b6494e3c2d4a80e7a8e2012f274987f6e8142
Signed-off-by: Jaehyun Kim <jeik01.kim@samsung.com>