Fix ZSTD_f_zstd1_magicless for small data

Fix ZSTD_f_zstd1_magicless for small data

* Fix `ZSTD_FRAMEHEADERSIZE_PREFIX` and `ZSTD_FRAMEHEADERSIZE_MIN` to
  take a `format` parameter, so it is impossible to get the wrong size.
* Fix the places that called `ZSTD_FRAMEHEADERSIZE_PREFIX` without
  taking the format into account, which is now impossible by design.
* Call `ZSTD_frameHeaderSize_internal()` with `dctx->format`.
* The added tests catch both bugs in `ZSTD_decompressFrame()`.

Fixes #1813.