fmtsort: don't out-of-bounds panic if there's a race condition

Applied upstream CL 191197 to mapelem.go. Cannot fully port to 1.11,
since MapRange started to be supported in 1.12, but we can still avoid
an index out-of-bounds panic.

Fixes #108.

fmtsort/mapelem.go

@@ -7,12 +7,16 @@ import "reflect"
 const brokenNaNs = false
 
 func mapElems(mapValue reflect.Value) ([]reflect.Value, []reflect.Value) {
-	key := make([]reflect.Value, mapValue.Len())
-	value := make([]reflect.Value, len(key))
+	// Note: this code is arranged to not panic even in the presence
+	// of a concurrent map update. The runtime is responsible for
+	// yelling loudly if that happens. See issue 33275.
+	n := mapValue.Len()
+	key := make([]reflect.Value, 0, n)
+	value := make([]reflect.Value, 0, n)
 	iter := mapValue.MapRange()
-	for i := 0; iter.Next(); i++ {
-		key[i] = iter.Key()
-		value[i] = iter.Value()
+	for iter.Next() {
+		key = append(key, iter.Key())
+		value = append(value, iter.Value())
 	}
 	return key, value
 }

fmtsort/mapelem_1.11.go

@@ -8,16 +8,16 @@ const brokenNaNs = true
 
 func mapElems(mapValue reflect.Value) ([]reflect.Value, []reflect.Value) {
 	key := mapValue.MapKeys()
-	value := make([]reflect.Value, len(key))
-	for i, k := range key {
+	value := make([]reflect.Value, 0, len(key))
+	for _, k := range key {
 		v := mapValue.MapIndex(k)
 		if !v.IsValid() {
 			// Note: we can't retrieve the value, probably because
 			// the key is NaN, so just do the best we can and
 			// add a zero value of the correct type in that case.
 			v = reflect.Zero(mapValue.Type().Elem())
 		}
-		value[i] = v
+		value = append(value, v)
 	}
 	return key, value
 }