drive: fix index out of bounds when parsing request local paths (#15517)

Fix the index out of bound panic when a request is made to the local
fileserver mux with a valid secret-token, but missing share name.

Example error:

    http: panic serving 127.0.0.1:40974: runtime error: slice bounds out of range [2:1]

Additionally, we document the edge case behavior of utilities that
this fileserver mux depends on.

Signed-off-by: Craig Hesling <craig@hesling.com>

drive/driveimpl/shared/pathutil.go

@@ -22,6 +22,9 @@ const (
 // CleanAndSplit cleans the provided path p and splits it into its constituent
 // parts. This is different from path.Split which just splits a path into prefix
 // and suffix.
+//
+// If p is empty or contains only path separators, CleanAndSplit returns a slice
+// of length 1 whose only element is "".
 func CleanAndSplit(p string) []string {
 	return strings.Split(strings.Trim(path.Clean(p), sepStringAndDot), sepString)
 }
@@ -38,6 +41,8 @@ func Parent(p string) string {
 }
 
 // Join behaves like path.Join() but also includes a leading slash.
+//
+// When parts are missing, the result is "/".
 func Join(parts ...string) string {
 	fullParts := make([]string, 0, len(parts))
 	fullParts = append(fullParts, sepString)