kernel/os: Assert that eventq is valid before put
kernel/os: Assert that eventq is valid before put

Trigger a crash if the application attempts to put an event on a null or
uninitialized event queue.  Without the assert, both of these conditions
can put the system in a strange state: the event "thinks" it is
enqueued, but it is not actually present in the eventq's list.  As a
consequence, the event never gets processed, and it is impossible to
queue the event again because it thinks it is already enqueued.

mlaz/mynewt-core,mlaz/mynewt-core,mlaz/mynewt-core,mlaz/mynewt-core,mlaz/mynewt-core