fix compile error ioselector_poll setLen(0) instead clear()

[Ivansete-status]

This is to avoid the following compilation issue:

[arnetheduck]

this error suggests two things:

• the poll implementation is not being tested -> a test configuration should be added that tests the poll implementation (by adding a ci test matrix option and choosing poll explicitly on linux)
• the wrong implementation is being used for android -> android should be using epoll, not sure why that is not enabled cc @cheatfate

[cheatfate]

It actually being tested for all the compilers and posix platforms we are testing.
https://github.com/status-im/nim-chronos/blob/master/chronos.nimble#L33

And its not wrong implementation being used for Android, because different projects using different Android BASE versions, we can't use all functions from epoll engine when Android 4.0 is being selected, there was number of issues encountered, possibly fixed, but never tested because i do not have ability to test on Android.

[cheatfate]

But i think this function call is not being used in tests and its why it never fails to compilation error...

[Ivansete-status]

@cheatfate - I'm adding the suggestion you gave me in DM so that we perform a proper data cleanup by using reset() in a seq: 88581c6
Thanks!

[arnetheduck]

fyi build error is due to status-im/nimbus-common-workflow@44f5bd1#r175656464

[arnetheduck]

we can't use all functions

hm, from what I know, libevent uses epoll on android so it's probably a matter of getting it up and running - cc @Ivansete-status this is probably something that's good to look into since epoll is much more efficient (and tested) in general - ie there may very well be stability issues with the poll selector that we're unaware of.

[arnetheduck]

for some reason the merge button broke on this one .. let's see if reopening helps