fix support for booting from rtfs on tape

lib/libmtar.lua

@@ -30,7 +30,7 @@ function mtar.iter(stream) -- table -- function -- Given buffer *stream*, return
  end
  return function()
   while remain > 0 do
-   remain=remain-#stream:read(math.min(remain,2048))
+   remain=remain-(#stream:read(math.min(remain,2048)) or "")
   end
   local version = 0
   local nlen = string.unpack(">I2", stream:read(2) or "\0\0")

module/rtfsboot.lua

@@ -2,7 +2,7 @@
 --#includepkglib "rtfs" "lib/rtfs.lua" "rtfs"
 do
  local a = computer.getBootAddress()
- if component.type(a) == "drive" then
+ if component.type(a) == "drive" or component.type(a) == "tape_drive" then
   local diskpart = require "diskpart"
   for k,v in ipairs(diskpart.getPartitions(a)) do
    if v[2] == "rtfs" and v[1] == computer.address():sub(1,8) .. "-boot" then