zos: fix potential event loop stall
zos: fix potential event loop stall

This is a port of 70002c80 to z/OS to fix the same potential issue that
could effectively enter an infinite loop (but not a busy loop) under
certain conditions when polling for events.

PR-URL: https://github.com/libuv/libuv/pull/2725
Reviewed-By: Fedor Indutny <2e687df1861dff46b1954016c7b2cad32e8742d4@gmail.com>
Reviewed-By: Santiago Gimeno <3f17e29e343c4cf8809fb0970d81d6d8ee88d6b3@gmail.com>
Reviewed-By: Jameson Nash <99e7f19e9f64df56d4e81dc00b8566e722cdd766@gmail.com>

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