Bug dont make unknown metadata ids fail

We had a pile of production errors last weekend when a presumable safe schema change was made adding some optional fields to an existing struct.  When these fields were actually delivered to profman it raised a Runtime error from the [maxid test in lib/rb/ext/struct_metadata.c](https://github.com/whitepages/thrift/compare/whitepages:master...bug_dont_make_unknown_metadata_ids_fail#diff-b8bebb51ac1068d9f9adee07a4cb2bf6L36)).

This PR removes that test returning NULL when the id is bigger than one we know of.  The call sites getFieldMetadataByID() have correct semantics to skip the field on NULL.