Fixes #6787

In ProposedAllocs the proposed alloc slice was being copied while its
contents were not. Since RemoveAllocs nils elements of the proposed
alloc slice and is called twice, it could panic on the second call when
erroneously accessing a nil'd alloc.

The fix is to not copy the proposed alloc slice and pass the slice
returned by the 1st RemoveAllocs call to the 2nd call, thus maintaining
the trimmed length.