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/fileserver.go

@@ -142,6 +142,10 @@ func (s *FileServer) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
+	if len(parts) < 2 {
+		w.WriteHeader(http.StatusBadRequest)
+		return
+	}
 	r.URL.Path = shared.Join(parts[2:]...)
 	share := parts[1]
 	s.sharesMu.RLock()