Was missing a check in validSymbol.

Fixes #23580.

Can wait for go1.11. Probably safe but the crash is only for
invalid input, so not worth the risk.

Change-Id: I51f88c5be35a8880536147d1fe5c5dd6798c29de
Reviewed-on: https://go-review.googlesource.com/90398Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>