add missing asserts to UserAllocatedEvent class
add missing asserts to UserAllocatedEvent class

Improves code quality by adding MBED_ASSERT to UserAllocatedEvent::delay
and UserAllocatedEvent::period function. This prevent changing already posted event.

mbedmicro/mbed,andcor02/mbed-os,kjbracey-arm/mbed,andcor02/mbed-os,kjbracey-arm/mbed,andcor02/mbed-os,mbedmicro/mbed,andcor02/mbed-os,kjbracey-arm/mbed,mbedmicro/mbed,mbedmicro/mbed,mbedmicro/mbed,kjbracey-arm/mbed,andcor02/mbed-os,andcor02/mbed-os