path/filepath: windows drive letter cannot be a digit

R=golang-dev, r
CC=golang-dev, mattn.jp
https://golang.org/cl/5885056

src/pkg/path/filepath/path_test.go

@@ -814,6 +814,7 @@ type VolumeNameTest struct {
 var volumenametests = []VolumeNameTest{
 	{`c:/foo/bar`, `c:`},
 	{`c:`, `c:`},
+	{`2:`, ``},
 	{``, ``},
 	{`\\\host`, ``},
 	{`\\\host\`, ``},

src/pkg/path/filepath/path_windows.go

@@ -35,9 +35,7 @@ func VolumeName(path string) (v string) {
 	}
 	// with drive letter
 	c := path[0]
-	if path[1] == ':' &&
-		('0' <= c && c <= '9' || 'a' <= c && c <= 'z' ||
-			'A' <= c && c <= 'Z') {
+	if path[1] == ':' && ('a' <= c && c <= 'z' || 'A' <= c && c <= 'Z') {
 		return path[:2]
 	}
 	// is it UNC