The equeue_chain function is supposed to unchain the event queue
from whatever queue it is chained to when passed a null target.
Internally, this is accomplished by just calling equeue_background
with null and letting the previously registered update function
clean up the chaining.
However, equeue_chain did not appropriately check for null, causing
it to unnecessarily allocate memory and leaving the update function
in a bad state. Fixed with a simple null check.
Added mbed-events from https://github.com/ARMMbed/mbed-events. Changes
from upstream:
- the whole code is licensed under the Apache license. Sources and
headers were updates with this information.
- removed the porting layers for Windows and FreeRTOS and the references
to these porting layers in equeue_platform.h.
- moved the TESTS directory in mbed-events to the TESTS directory of
mbed-os.