AP MLD: Add NULL check for mld pointer during MLD link removal

AP MLD: Add NULL check for mld pointer during MLD link removal

MLD control interface creation failed randomly because the existing
control interface was not properly cleaned up. During the failure case
handling, the memory allocated for hapd->mld is freed in
hostapd_bss_setup_multi_link(). Subsequently, when performing MLD link
removal during the interface down, hostapd_mld_remove_link() attempts to
access the mld pointer without NULL check, causing a segmentation fault.

To prevent this issue, add a validation to check if the mld pointer
is NULL before accessing it.

Signed-off-by: Govindaraj Saminathan <quic_gsaminat@quicinc.com>