extmod/uasyncio: Truncate negative sleeps to 0.

Otherwise a task that continuously awaits on a large negative sleep can
monopolise the scheduler (because its wake time is always less than
everything else in the pairing heap).

Signed-off-by: Damien George <damien@micropython.org>

extmod/uasyncio/core.py

@@ -53,7 +53,7 @@ class SingletonGenerator:
 # Use a SingletonGenerator to do it without allocating on the heap
 def sleep_ms(t, sgen=SingletonGenerator()):
     assert sgen.state is None
-    sgen.state = ticks_add(ticks(), t)
+    sgen.state = ticks_add(ticks(), max(0, t))
     return sgen