compiler: don't advance past unexpected semicolon

We've already read the unexpected semicolon, so advancing again causes
us to skip the next token, causing future errors to be out of sync.

Change-Id: Id83776a31996dbfd4734420fd3e8b4cfcc14d2f0
Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/274439
Trust: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>
Reviewed-by: Than McIntosh <thanm@google.com>
1 file changed