Fix data corruption in niche use case

[terrelln]

• Extract the overflow correction into a helper function.
• Load the dictionary ZSTD_CHUNKSIZE_MAX = 512 MB bytes at a time
  and overflow correct between each chunk.
• Add a test case that made overflow asserts fail, and now passes.
  I couldn't figure out how to make a fast test that causes data corruption.

Data corruption could happen when all these conditions are true:

• You are using multithreading mode
• Your overlap size is >= 512 MB (implies window size >= 512 MB)
• You are using a strategy >= ZSTD_btlazy
• You are compressing more than 4 GB

The problem is that when loading a large dictionary we don't do
overflow correction. We can only load 512 MB at a time, and may
need to do overflow correction before each chunk.

Fixes #1653.