fmt: one bad index shouldn't spoil them all

In an indexed verb such as %[3]d, if the index is out of range, don't
skip processing the rest of the verbs. The bug was that the bad
index set a bit for the whole format instead of just the verb.

Ok for 1.2 because this is a bug in a 1.2 feature.

Fixes #6434

R=golang-dev, adg
CC=golang-dev
https://golang.org/cl/13632058

src/pkg/fmt/fmt_test.go

@@ -589,6 +589,8 @@ var reorderTests = []struct {
 	{"%3.[2]d", SE{7}, "%!d(BADINDEX)"},
 	{"%.[2]d", SE{7}, "%!d(BADINDEX)"},
 	{"%d %d %d %#[1]o %#o %#o %#o", SE{11, 12, 13}, "11 12 13 013 014 015 %!o(MISSING)"},
+	{"%[5]d %[2]d %d", SE{1, 2, 3}, "%!d(BADINDEX) 2 3"},
+	{"%d %[3]d %d", SE{1, 2}, "1 %!d(BADINDEX) 2"}, // Erroneous index does not affect sequence.
 }
 
 func TestReorder(t *testing.T) {

src/pkg/fmt/print.go

@@ -118,7 +118,7 @@ type pp struct {
 	value reflect.Value
 	// reordered records whether the format string used argument reordering.
 	reordered bool
-	// goodArgNum records whether all reordering directives were valid.
+	// goodArgNum records whether the most recent reordering directive was valid.
 	goodArgNum bool
 	runeBuf    [utf8.UTFMax]byte
 	fmt        fmt
@@ -1036,7 +1036,7 @@ func intFromArg(a []interface{}, argNum int) (num int, isInt bool, newArgNum int
 // up to the closing paren, if present, and whether the number parsed
 // ok. The bytes to consume will be 1 if no closing paren is present.
 func parseArgNumber(format string) (index int, wid int, ok bool) {
-	// Find closing parenthesis
+	// Find closing bracket.
 	for i := 1; i < len(format); i++ {
 		if format[i] == ']' {
 			width, ok, newi := parsenum(format, 1, i)
@@ -1070,8 +1070,8 @@ func (p *pp) doPrintf(format string, a []interface{}) {
 	argNum := 0         // we process one argument per non-trivial format
 	afterIndex := false // previous item in format was an index like [3].
 	p.reordered = false
-	p.goodArgNum = true
 	for i := 0; i < end; {
+		p.goodArgNum = true
 		lasti := i
 		for i < end && format[i] != '%' {
 			i++