`ZDICT_finalizeDictionary()` had a flipped comparison.
I also allowed `dictBufferCapacity == dictContentSize`.
It might be the case that the user wants to fill the dictionary
completely up, and then let zstd take exactly the space it needs
for the entropy tables.
In some cases a custom dictionary building algorithm tailored for a specific
input can be more effective than the one produced by `ZDICT_trainFromBuffer`,
but with the current API it's not possible encode the entropy tables into the
custom-built dictionary.
This commit extracts the logic to add entropy tables to a dictionary from
`ZDICT_trainFromBuffer` and exposes it as a function
`ZDICT_addEntropyTablesFromBuffer`.
