fmt: empty byte slices should print nothing in hex

The documentation is clear that formats like %02x applied to a
byte slice are per-element, so the result should be nothing if the
slice is empty. It's not, because the top-level padding routine is called.
It shouldn't be: the loop does the padding for us.

Fixes #10430.

Change-Id: I04ea0e804c0f2e70fff3701e5bf22acc90e890da
Reviewed-on: https://go-review.googlesource.com/8864Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/fmt/fmt_test.go

@@ -394,6 +394,8 @@ var fmtTests = []struct {
 	{"%v", &slice, "&[1 2 3 4 5]"},
 	{"%v", &islice, "&[1 hello 2.5 <nil>]"},
 	{"%v", &bslice, "&[1 2 3 4 5]"},
+	{"%v", []byte{1}, "[1]"},
+	{"%v", []byte{}, "[]"},
 
 	// complexes with %v
 	{"%v", 1 + 2i, "(1+2i)"},
@@ -447,6 +449,12 @@ var fmtTests = []struct {
 	{"%d", []int{1, 2, 15}, `[1 2 15]`},
 	{"%d", []byte{1, 2, 15}, `[1 2 15]`},
 	{"%q", []string{"a", "b"}, `["a" "b"]`},
+	{"% 02x", []byte{1}, "01"},
+	{"% 02x", []byte{1, 2, 3}, "01 02 03"},
+	// Special care for empty slices.
+	{"%x", []byte{}, ""},
+	{"%02x", []byte{}, ""},
+	{"% 02x", []byte{}, ""},
 
 	// renamings
 	{"%v", renamedBool(true), "true"},

src/fmt/format.go

@@ -335,7 +335,7 @@ func (f *fmt) fmt_sbx(s string, b []byte, digits string) {
 		}
 		buf = append(buf, digits[c>>4], digits[c&0xF])
 	}
-	f.pad(buf)
+	f.buf.Write(buf)
 }
 
 // fmt_sx formats a string as a hexadecimal encoding of its bytes.