math/big: avoid panic in float.Text with negative prec

Fixes #15918

Change-Id: I4b434aed262960a2e6c659d4c2296fbf662c3a52
Reviewed-on: https://go-review.googlesource.com/23633
Run-TryBot: Robert Griesemer <gri@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/math/big/floatconv_test.go

@@ -290,6 +290,11 @@ func TestFloat64Text(t *testing.T) {
 		// Issue 2625.
 		{383260575764816448, 'f', 0, "383260575764816448"},
 		{383260575764816448, 'g', -1, "3.8326057576481645e+17"},
+
+		// Issue 15918.
+		{1, 'f', -10, "1"},
+		{1, 'f', -11, "1"},
+		{1, 'f', -12, "1"},
 	} {
 		// The test cases are from the strconv package which tests float64 values.
 		// When formatting values with prec = -1 (shortest representation),

src/math/big/ftoa.go

@@ -41,8 +41,11 @@ import (
 // x.Prec() mantissa bits.
 // The prec value is ignored for the 'b' or 'p' format.
 func (x *Float) Text(format byte, prec int) string {
-	const extra = 10 // TODO(gri) determine a good/better value here
-	return string(x.Append(make([]byte, 0, prec+extra), format, prec))
+	cap := 10 // TODO(gri) determine a good/better value here
+	if prec > 0 {
+		cap += prec
+	}
+	return string(x.Append(make([]byte, 0, cap), format, prec))
 }
 
 // String formats x like x.Text('g', 10).