This is a more logical place to clear the KeyboardInterrupt traceback,
right before it is set as a pending exception. The clearing is also
optimised from a function call to a simple store of NULL.
This function is tightly coupled to the state and behaviour of the
scheduler, and is a core part of the runtime: to schedule a pending
exception. So move it there.
Otherwise mp_interrupt_char will have a value of zero on start up (because
it's in the BSS) and a KeyboardInterrupt may be raised during start up.
For example this can occur if there is a UART attached to the REPL which
sends spurious null bytes when the device turns on.
