Fixes #15055.

Updates exprfmt printing using fmt verb "%v" to check that n.Left
is non-nil before attempting to print it, otherwise we'll print
the nodes in the list using verb "%.v".

Credit to @mdempsky for this approach and for finding
the root cause of the issue.

Change-Id: I20a6464e916dc70d5565e145164bb9553e5d3865
Reviewed-on: https://go-review.googlesource.com/25361Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>