math/big: Float.Text panics with prec < -11

[ALTree]

$ gotip version
go version devel +a93a124 Tue May 31 15:33:01 2016 +0200 linux/amd64

The documentation of the (x *Float) Text(format byte, prec int) string function says

A negative precision selects the smallest number of decimal digits necessary to identify the value x uniquely.

This is true for -11 < prec < 0, but for prec < -10 the function panics instead:

package main

import (
    "fmt"
    "math/big"
)

func main() {
    z := new(big.Float).SetRat(big.NewRat(1, 3))
    fmt.Println(z.Text('f', -11))

}

playground link.

panic: runtime error: makeslice: cap out of range

goroutine 1 [running]:
panic(0x1886a0, 0x1040a138)
    /usr/local/go/src/runtime/panic.go:481 +0x700
math/big.(*Float).Text(0x10427f70, 0x10430266, 0xfffffff5, 0x0, 0x0, 0x0)
    /usr/local/go/src/math/big/ftoa.go:45 +0x60
main.main()
    /tmp/sandbox979738453/main.go:10 +0xa0

[ALTree]

I have a simple patch, will send for go1.8.

[griesemer]

If it's simple enough, send it to me now, please.

[ALTree]

Sent https://go-review.googlesource.com/#/c/23633/

[gopherbot]

CL https://golang.org/cl/23633 mentions this issue.