internal/number: possible out of range error avoiding

Case when r.Increment > 0 and r.IncrementScale == len(scales) in *Decimal.ConvertFloat method is taken into account.
It led to unexpected out of range panic.

fixes golang/go#42147

Change-Id: Ic26e67010b766bdbd322a3853489f6d1ecb0dcfc
Reviewed-on: https://go-review.googlesource.com/c/text/+/265021
Reviewed-by: Marcel van Lohuizen <mpvl@golang.org>
Trust: Russ Cox <rsc@golang.org>

internal/number/decimal.go

diff --git a/internal/number/decimal.go b/internal/number/decimal.go
index 9b4035e..cb656db 100644
--- a/internal/number/decimal.go
+++ b/internal/number/decimal.go
@@ -379,7 +379,7 @@
 	if r.Increment > 0 {
 		scale := int(r.IncrementScale)
 		mult := 1.0
-		if scale > len(scales) {
+		if scale >= len(scales) {
 			mult = math.Pow(10, float64(scale))
 		} else {
 			mult = scales[scale]

internal/number/decimal_test.go

diff --git a/internal/number/decimal_test.go b/internal/number/decimal_test.go
index 97c7e25..670e806 100644
--- a/internal/number/decimal_test.go
+++ b/internal/number/decimal_test.go
@@ -248,6 +248,10 @@
 	inc0_05 := RoundingContext{Increment: 5, IncrementScale: 2}
 	inc0_05.SetScale(2)
 	inc50 := RoundingContext{Increment: 50}
+	incScaleEqualToScalesLen := RoundingContext{Increment: 1, IncrementScale: 0}
+	if len(scales) <= math.MaxUint8 {
+		incScaleEqualToScalesLen.IncrementScale = uint8(len(scales))
+	}
 	prec3 := RoundingContext{}
 	prec3.SetPrecision(3)
 	roundShift := RoundingContext{DigitShift: 2, MaxFractionDigits: 2}
@@ -309,6 +313,7 @@
 		{math.Inf(-1), inc50, "-Inf"},
 		{math.NaN(), inc50, "NaN"},
 		{"clearly not a number", scale2, "NaN"},
+		{0.0, incScaleEqualToScalesLen, "0"},
 	}
 	for _, tc := range testCases {
 		var d Decimal