FFI: fix deadlock in hyper_executor::poll_next #2239

Job	Run time
Check Style	7s
Verify hyper.h is up to date	38s
Check MSRV	1m 0s
Test C API (FFI)	41s
features	31s
Test with Miri	1m 22s
udeps	35s
Test stable on ubuntu-latest	1m 46s
Test stable on windows-latest	4m 24s
Test stable on macOS-latest	2m 25s
Test beta on ubuntu-latest	1m 35s
Test beta on windows-latest	3m 3s
Test beta on macOS-latest	2m 42s
Test nightly on ubuntu-latest	2m 0s
Test nightly on windows-latest	2m 47s
Test nightly on macOS-latest	1m 39s
Check exposed types	31s
Build docs	18s
CI is green	1s
	28m 5s

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