Improve error checking in deflatePrime()

Improve error checking in deflatePrime()

This is a small follow-up commit to the previous commit.

ok mbuhl millert

commit 4346a16853e19b45787ce933666026903fb8f3f8
Author: Mark Adler <madler@alumni.caltech.edu>
Date:   Tue Apr 17 22:44:41 2018 -0700

    Assure that the number of bits for deflatePrime() is valid.

https://github.com/madler/zlib/commit/4346a16853e19b45787ce933666026903fb8f3f8