Corrected refcounting mistakes with union types and rttc and build a better error message.

Corrected two refcounting mistakes in check_rtt_compatibility_inl
(one forgotten free_lpctype and one free_lpctype called to early).
When determining the full type of an array don't abort on the
first unfitting element. Ignore zeroes in the array as they don't
contribute anything to the resulting type.