Rollup merge of #147035 - joboet:extract_if_debug, r=tgross35

alloc: fix `Debug` implementation of `ExtractIf`

I noticed this while reviewing rust-lang/rust#141032. Calling `get` on the inner `Vec` never returns `Some` as the `Vec`'s length is temporarily set to zero while the `ExtractIf` exists.

Author: matthiaskrgr