Limit the number of elements in a consdiff hash line.

This avoids performing and then freeing a lot of small mallocs() if the hash line has too many elements.

Fixes one case of bug #40472 (closed); resolves OSS-Fuzz 38363. Bugfix on 0.3.1.1-alpha when the consdiff parsing code was introduced.

I'm opening this fix against 0.3.5, but I do think it's safe not to backport it.