go.exp/go/types: don't crash when reporting too many arguments

A test case is already present in expr3.src:322 but disabled
for another reason at the moment.

Also: use tuple accessor 'At' to get to tuple vars since the
the tuple may be empty and represented by a nil *Tuple.

R=adonovan
CC=golang-dev
https://golang.org/cl/9368045
2 files changed