cmd/compile: fix off-by-1 in getr

Introduced by (and missed during code review of) golang.org/cl/19847.

Change-Id: I03b76f36e5da69c31730380592dfa1c32570e17f
Reviewed-on: https://go-review.googlesource.com/19912
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Robert Griesemer <gri@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/cmd/compile/internal/gc/lex.go

@@ -1873,7 +1873,7 @@ redo:
 		// The string conversion here makes a copy for passing
 		// to fmt.Printf, so that buf itself does not escape and
 		// can be allocated on the stack.
-		Yyerror("illegal UTF-8 sequence % x", string(buf[:i+1]))
+		Yyerror("illegal UTF-8 sequence % x", string(buf[:i]))
 	}
 
 	if r == BOM {