strconv: fix round up corner case

Comment described the correct condition
but the code did not implement it.

Fixes #2625.

R=remyoudompheng
CC=golang-dev
https://golang.org/cl/5530082

src/pkg/strconv/ftoa.go

@@ -241,7 +241,7 @@ func roundShortest(d *decimal, mant uint64, exp int, flt *floatInfo) {
 
 		// Okay to round up if upper has a different digit and
 		// either upper is inclusive or upper is bigger than the result of rounding up.
-		okup := m != u && (inclusive || i+1 < upper.nd)
+		okup := m != u && (inclusive || m+1 < u || i+1 < upper.nd)
 
 		// If it's okay to do either, then round to the nearest one.
 		// If it's okay to do only one, do it.

src/pkg/strconv/ftoa_test.go

@@ -123,6 +123,10 @@ var ftoatests = []ftoaTest{
 	{2.2250738585072012e-308, 'g', -1, "2.2250738585072014e-308"},
 	// http://www.exploringbinary.com/php-hangs-on-numeric-value-2-2250738585072011e-308/
 	{2.2250738585072011e-308, 'g', -1, "2.225073858507201e-308"},
+
+	// Issue 2625.
+	{383260575764816448, 'f', 0, "383260575764816448"},
+	{383260575764816448, 'g', -1, "3.8326057576481645e+17"},
 }
 
 func TestFtoa(t *testing.T) {