Some invalid input may be parsed so that the length of an opaque
subpacket turns out to be 0. In such cases, arrange for a
StructuralError to be returned indicating truncation.
Found using gofuzz.
Fixesgolang/go#11503
Change-Id: Ib9ce8c604f35a31f852adfcd56a22dfd143a9443
Reviewed-on: https://go-review.googlesource.com/12634
Reviewed-by: Adam Langley <agl@golang.org>