subsys/random: Add _ASSERT() test on returned device_get_binding
subsys/random: Add _ASSERT() test on returned device_get_binding

If there is a build setup problem where a device driver has not been
setup for the entropy driver then the call to device_get_binding()
will return a NULL value and the code will continue to use this NULL
value. The result is a hard fault later in code execution.

Note that CONFIG_ASSERT is by default off so one has to turn this
configuration on to catch this problem.

Signed-off-by: David Leach <98707a1e43c521c5b5b81e36afef696dde6831a3@nxp.com>

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