Fix Data Corruption Bug when Streaming with an Attached Dict in HC Mode

Fix Data Corruption Bug when Streaming with an Attached Dict in HC Mode

This diff fixes an issue in which we failed to clear the `dictCtx` in HC
compression. The `dictCtx` is not supposed to be used when an `extDict` is
present: matches found in the `dictCtx` do not account for the presence of an
`extDict` segment, and their offsets are therefore miscalculated when one is
present. This can lead to data corruption.

This diff clears the `dictCtx` whenever setting an `extDict`.

This issue was uncovered by @terrelln's fuzzing work.