Throw on skipping an invalid type.

Summary:
Certain values (e.g.) T_STOP should not appear as a skip type. Allowing them to
can cause thrift to loop unboundedly.

Reviewed By: spalamarchuk

Differential Revision: D15102451

fbshipit-source-id: c08d52f44f37e9c212d3480233ac217105586c9f