net: lib: dhcpv4: Add check for input parameter of echo_reply_handler

For station and internal AP coexist case, station connected to
external AP, ping from station to external AP cause cpu hang.

Internal AP dhcpv4 server register handler echo_reply_handler
on icmp handler by net_icmp_init_ctx for dhcp server snoop
feature. Ping also register handler handle_ipv4_echo_reply on
icmp handler for ping cmd. If no external station connect to
internal AP, input parameter ‘user_data’ of function
echo_reply_handler is NULL. When we trigger ping process,
icmp_call_handlers fetch all handlers from icmp handler
if receive any ICMP packet, so echo_reply_handler be called,
but input parameter is NULL, cause CPU hang.
Add input parameters check to fix this issue.

Signed-off-by: Rex Chen <rex.chen_1@nxp.com>