Fix double-drop in `Vec::from_iter(vec.into_iter())` specialization when items drop during panic
This fixes the double-drop but it leaves a behavioral difference compared to the default implementation intact: In the default implementation the source and the destination vec are separate objects, so they get dropped separately. Here they share an allocation and the latter only exists as a pointer into the former. So if dropping the former panics then this fix will leak more items than the default implementation would. Is this acceptable or should the specialization also mimic the default implementation's drops-during-panic behavior?
Fixes#83618
`@rustbot` label T-libs-impl
542f441d44