math/big: avoid MinExp exponent wrap-around in 'x' Text format

Fixes #34343.

Change-Id: I74240c8f431f6596338633a86a7a5ee1fce70a65
Reviewed-on: https://go-review.googlesource.com/c/go/+/196057
Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/math/big/floatconv_test.go

@@ -536,6 +536,10 @@ func TestFloatText(t *testing.T) {
 		{"-8191.53125", ToNegativeInf, 53, 'x', 4, "-0x1.fff9p+12"},
 		{"8191.53125", ToPositiveInf, 53, 'x', 4, "0x1.fff9p+12"},
 		{"-8191.53125", ToPositiveInf, 53, 'x', 4, "-0x1.fff8p+12"},
+
+		// issue 34343
+		{"0x.8p-2147483648", ToNearestEven, 4, 'p', -1, "0x.8p-2147483648"},
+		{"0x.8p-2147483648", ToNearestEven, 4, 'x', -1, "0x1p-2147483649"},
 	} {
 		f, _, err := ParseFloat(test.x, 0, test.prec, ToNearestEven)
 		if err != nil {

src/math/big/ftoa.go

@@ -384,7 +384,7 @@ func (x *Float) fmtX(buf []byte, prec int) []byte {
 	case w > n:
 		m = nat(nil).shr(m, w-n)
 	}
-	exp := x.exp - 1
+	exp64 := int64(x.exp) - 1 // avoid wrap-around
 
 	hm := m.utoa(16)
 	if debugFloat && hm[0] != '1' {
@@ -397,7 +397,6 @@ func (x *Float) fmtX(buf []byte, prec int) []byte {
 	}
 
 	buf = append(buf, 'p')
-	exp64 := int64(exp)
 	if exp64 >= 0 {
 		buf = append(buf, '+')
 	} else {