Fix Path.extract_build* functions when applied on the build_dir itself

This should fix #20

src/path.ml

@@ -309,24 +309,28 @@ let is_in_build_dir t =
 let extract_build_context t =
   if is_local t && String.is_prefix t ~prefix:build_prefix then
     let i = String.length build_prefix in
-    match String.index_from t i '/' with
-    | exception _ -> None
-    | j ->
-      Some
-        (String.sub t ~pos:i ~len:(j - i),
-         String.sub t ~pos:(j + 1) ~len:(String.length t - j - 1))
+    let j =
+      match String.index_from t i '/' with
+      | exception _ -> String.length t
+      | j -> j
+    in
+    Some
+      (String.sub t ~pos:i ~len:(j - i),
+       String.sub t ~pos:(j + 1) ~len:(String.length t - j - 1))
   else
     None
 
 let extract_build_context_dir t =
   if is_local t && String.is_prefix t ~prefix:build_prefix then
     let i = String.length build_prefix in
-    match String.index_from t i '/' with
-    | exception _ -> None
-    | j ->
-      Some
-        (String.sub t ~pos:0 ~len:j,
-         String.sub t ~pos:(j + 1) ~len:(String.length t - j - 1))
+    let j =
+      match String.index_from t i '/' with
+      | exception _ -> String.length t
+      | j -> j
+    in
+    Some
+      (String.sub t ~pos:0 ~len:j,
+       String.sub t ~pos:(j + 1) ~len:(String.length t - j - 1))
   else
     None