Fix double unmount

[developit]

This should fix #4104.

I believe the issue is that unmount() is triggered for the VNode at oldChildren[i], but that VNode remains in oldChildren. When we hit the end of diffChildren(), a loop invokes unmount() on every non-null vnode in oldChildren, which still includes the eagerly-unmounted VNode.

I didn't have a chance to add a test as I'm doing this from GH web, if someone gets around to it before me we can likely just use the example from #4104.

[github-actions]

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[github-actions]

Size Change: +8 B (0%)

Total Size: 57.3 kB

Filename	Size	Change
dist/preact.js	4.36 kB	+1 B
dist/preact.min.js	4.4 kB	+2 B (0%)
dist/preact.min.module.js	4.4 kB	+2 B (0%)
dist/preact.min.umd.js	4.42 kB	+1 B
dist/preact.module.js	4.39 kB	+1 B
dist/preact.umd.js	4.43 kB	+1 B

ℹ️ View Unchanged

Filename	Size	Change
compat/dist/compat.js	3.95 kB	0 B
compat/dist/compat.module.js	3.87 kB	0 B
compat/dist/compat.umd.js	4.01 kB	0 B
debug/dist/debug.js	3.64 kB	0 B
debug/dist/debug.module.js	3.62 kB	0 B
debug/dist/debug.umd.js	3.71 kB	0 B
devtools/dist/devtools.js	232 B	0 B
devtools/dist/devtools.module.js	240 B	0 B
devtools/dist/devtools.umd.js	315 B	0 B
hooks/dist/hooks.js	1.53 kB	0 B
hooks/dist/hooks.module.js	1.56 kB	0 B
hooks/dist/hooks.umd.js	1.62 kB	0 B
jsx-runtime/dist/jsxRuntime.js	360 B	0 B
jsx-runtime/dist/jsxRuntime.module.js	326 B	0 B
jsx-runtime/dist/jsxRuntime.umd.js	441 B	0 B
test-utils/dist/testUtils.js	453 B	0 B
test-utils/dist/testUtils.module.js	454 B	0 B
test-utils/dist/testUtils.umd.js	536 B	0 B

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[coveralls]

coverage: 99.563%. remained the same when pulling b4dc892 on bugfix/double-unmount into 2e89591 on main.

[andrewiggins]

Added a test.

Something fun I noticed: You can't observe this bug using lifecycles cuz on the first unmount, we clear _component which prevents re-entry into the part of unmount that invokes componentWillUnmount.

[marvinhagemeister]

Nice catch 👍