The UNCOMPRESS() function was not marked as maybe_null, even though
  it returns NULL on invalid data. This confused the optimizer.


mysql-test/r/func_compress.result:
  Add new results
mysql-test/t/func_compress.test:
  Add new regression test
sql/item_strfunc.h:
  Set maybe_null within Item_func_uncompress::fix_length_and_dec()