Fix Appender extend

Fixes the usage of "GC.extend" in Appender.

Long story short: Don't extend allocations you don't own: Even if it succeeds:
- It doesn't mean you own the extended space (let alone the data "up to" the start of the extension).
- You don't know the underlying memory layout (eg `BlkAttr.APPENDABLE`).

Fixes 12428, as well as the snippet submitted in 9092.

This edit changes the indentation, so I suggest viewing the diff without whites:
https://github.com/D-Programming-Language/phobos/pull/2076/files?w=1