Fix stack trace when LuaJIT FFI data is present

The LuaJIT FFI introduces a new Lua type `cdata`.  It is not handled specifically by MobDebug, producing an error in the Stack Window (and no stack)

This patch simply adds `cdata` to the `badtype` blacklist.  FFI types now print something like "cdata<struct foo>: 0x40a44d30".  If people want more informative cdata, they can implement a metamethod.

This patch is released under the MIT license.