net: ieee802154: net_udp_get_hdr() might return NULL
net: ieee802154: net_udp_get_hdr() might return NULL

Assertions should only be used to check invariants.  Things that may
change value in runtime are better left to proper checks.

Signed-off-by: Leandro Pereira <51c7437a02b34df685e6b4ecb2e9bca23efe2e1a@intel.com>

ldts/zephyr,GiulianoFranchetto/zephyr,explora26/zephyr,mbolivar/zephyr,mbolivar/zephyr,kraj/zephyr,zephyrproject-rtos/zephyr,zephyriot/zephyr,kraj/zephyr,finikorg/zephyr,galak/zephyr,Vudentz/zephyr,explora26/zephyr,Vudentz/zephyr,galak/zephyr,nashif/zephyr,mbolivar/zephyr,galak/zephyr,punitvara/zephyr,nashif/zephyr,ldts/zephyr,zephyrproject-rtos/zephyr,punitvara/zephyr,zephyriot/zephyr,mbolivar/zephyr,nashif/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,zephyriot/zephyr,finikorg/zephyr,ldts/zephyr,kraj/zephyr,nashif/zephyr,ldts/zephyr,Vudentz/zephyr,kraj/zephyr,nashif/zephyr,explora26/zephyr,GiulianoFranchetto/zephyr,punitvara/zephyr,zephyrproject-rtos/zephyr,punitvara/zephyr,galak/zephyr,zephyriot/zephyr,Vudentz/zephyr,GiulianoFranchetto/zephyr,Vudentz/zephyr,kraj/zephyr,explora26/zephyr,GiulianoFranchetto/zephyr,punitvara/zephyr,Vudentz/zephyr,finikorg/zephyr,GiulianoFranchetto/zephyr,explora26/zephyr,galak/zephyr,ldts/zephyr,zephyriot/zephyr,mbolivar/zephyr