The existing implementation panics on malformed input of an empty
string. strconv.Unquote validates the length of the inputs, but calling
UnquoteChar directly with an empty string leads to a panic, so add a
check for length. Also, add a test to go/constant to ensure that
MakeFromLiteral does not panic on malformed input such as
"const x = ''".

Change-Id: I4217e38db48a09a21ec414bbfb3087709da62904
Reviewed-on: https://go-review.googlesource.com/116215
Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>