Fix `ForeverStack::find_starting_point` output parameter #8317

Job	Run time
build-and-check-ubuntu-64bit	25m 17s
build-and-check-ubuntu-64bit-glibcxx	26m 36s
build-and-check-asan	28m 22s
build-and-check-clang-macos	50m 58s
build-and-check-gcc-5	30m 4s
build-and-check-ubuntu-32bit	25m 21s
	3h 6m 38s

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