fmt: fix padding for 0 precision 0 integer value formatting

Fixes #14924

Change-Id: I098ef973e2cad76a121704492758c2971a9b55f3
Reviewed-on: https://go-review.googlesource.com/20920
Run-TryBot: Rob Pike <r@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Rob Pike <r@golang.org>

src/fmt/fmt_test.go

@@ -345,6 +345,7 @@ var fmtTests = []struct {
 	{"%x", ^uint32(0), "ffffffff"},
 	{"%X", ^uint64(0), "FFFFFFFFFFFFFFFF"},
 	{"%.20b", 7, "00000000000000000111"},
+	{"%6.0d", 0, "      "},
 	{"%10d", 12345, "     12345"},
 	{"%10d", -12345, "    -12345"},
 	{"%+10d", 12345, "    +12345"},

src/fmt/format.go

@@ -192,8 +192,11 @@ func (f *fmt) fmt_unicode(u uint64) {
 
 // fmt_integer formats signed and unsigned integers.
 func (f *fmt) fmt_integer(u uint64, base int, isSigned bool, digits string) {
-	// precision of 0 and value of 0 means "print nothing"
+	// Precision of 0 and value of 0 means "print nothing" but padding.
 	if f.precPresent && f.prec == 0 && u == 0 {
+		if f.widPresent {
+			f.writePadding(f.wid)
+		}
 		return
 	}