strings: fix minor bug in LastIndexFunc

R=r, rsc
CC=golang-dev
https://golang.org/cl/2250042

src/pkg/strings/strings.go

@@ -463,7 +463,7 @@ func lastIndexFunc(s string, f func(r int) bool, truth bool) int {
 				}
 			}
 			if start < 0 {
-				return -1
+				start = 0
 			}
 			var wid int
 			rune, wid = utf8.DecodeRuneInString(s[start:end])

src/pkg/strings/strings_test.go

@@ -514,6 +514,7 @@ var indexFuncTests = []IndexFuncTest{
 	IndexFuncTest{"\xc0☺\xc0\xc0", not(isValidRune), 0, 5},
 	IndexFuncTest{"ab\xc0a\xc0cd", not(isValidRune), 2, 4},
 	IndexFuncTest{"a\xe0\x80cd", not(isValidRune), 1, 2},
+	IndexFuncTest{"\x80\x80\x80\x80", not(isValidRune), 0, 3},
 }
 
 func TestIndexFunc(t *testing.T) {