fix edge case where io.open(whatever):read() returns nil

lib/libmtar.lua

@@ -24,7 +24,7 @@ end
 function mtar.iter(stream) -- table -- function -- Given buffer *stream*, returns an iterator suitable for use with *for* that returns, for each iteration, the file name, a function to read from the file, and the length of the file.
  local remain = 0
  local function read(n)
-  local rb = stream:read(math.min(n,remain))
+  local rb = stream:read(math.min(n,remain)) or ""
   remain = remain - rb:len()
   return rb
  end