During the CONNECT_3GPP_CONTEXT_STEP_LAST step,
connect_3gpp_context_step() conditionally creates and populates a
MMBearerConnectResult object into the GSimpleAsyncResult object when the
ipv4_config field of the Connect3gppContext struct is set. That assumes
the ipv4_config field is always initialized in
connect_dhcp_check_ready() during the
CONNECT_3GPP_CONTEXT_STEP_IP_CONFIG step. Instead of having such an
assumption, this patch modifies connect_3gpp to always initialize
the ipv4_config field, such that connect_3gpp_context_step() always
populates a MMBearerConnectResult object into the GSimpleAsyncResult
object.