path/filepath: fix Join with Windows drive letter

Join("C:", "", "b") must return relative path "C:b"

Fixes #26953

Change-Id: I2f843ce3f9f18a1ce0e2d0f3a15233f237992776
Reviewed-on: https://go-review.googlesource.com/129758
Run-TryBot: Emmanuel Odeke <emm.odeke@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Alex Brainman <alex.brainman@gmail.com>

src/path/filepath/path_test.go

@@ -271,6 +271,10 @@ var winjointests = []JoinTest{
 	{[]string{`C:`, `a`}, `C:a`},
 	{[]string{`C:`, `a\b`}, `C:a\b`},
 	{[]string{`C:`, `a`, `b`}, `C:a\b`},
+	{[]string{`C:`, ``, `b`}, `C:b`},
+	{[]string{`C:`, ``, ``, `b`}, `C:b`},
+	{[]string{`C:`, ``}, `C:.`},
+	{[]string{`C:`, ``, ``}, `C:.`},
 	{[]string{`C:.`, `a`}, `C:a`},
 	{[]string{`C:a`, `b`}, `C:a\b`},
 	{[]string{`C:a`, `b`, `d`}, `C:a\b\d`},

src/path/filepath/path_windows.go

@@ -134,7 +134,14 @@ func joinNonEmpty(elem []string) string {
 	if len(elem[0]) == 2 && elem[0][1] == ':' {
 		// First element is drive letter without terminating slash.
 		// Keep path relative to current directory on that drive.
-		return Clean(elem[0] + strings.Join(elem[1:], string(Separator)))
+		// Skip empty elements.
+		i := 1
+		for ; i < len(elem); i++ {
+			if elem[i] != "" {
+				break
+			}
+		}
+		return Clean(elem[0] + strings.Join(elem[i:], string(Separator)))
 	}
 	// The following logic prevents Join from inadvertently creating a
 	// UNC path on Windows. Unless the first element is a UNC path, Join