fixed a few crashes resulting from the fs library

modules/lib/fs.lua

@@ -42,13 +42,15 @@ do
  function fs.open(p,m)
   local _,d,p = fs.resolve(p)
   local d = fT[d]
-  local f,C=d.open(p,m),hT._c
-  if f then
-   hT._c = C + 1
-   hT[C] = {d,f}
-   return C
+  if d then
+   local f,C=d.open(p,m),hT._c
+   if f then
+    hT._c = C + 1
+    hT[C] = {d,f}
+    return C
+   end
+   return false
   end
-  return false
  end
  function fs.close(h)
   if hT[h] then
@@ -99,7 +101,9 @@ do
  end
  function fs.exists(s)
   local _,d,p = fs.resolve(s)
-  return fT[d].exists(p)
+  if d then
+   return fT[d].exists(p)
+  end
  end
  function fs.isdir(s)
   local _,d,p = fs.resolve(s)