Two bug fixes in mdict_free

A heap-allocated multidictionary should be freed by calling
mdict_free.  However, while this function does free the contents of
the dictionary, it neglects to free the dictionary itself.

There's also a second bug, which is that if a multidictionary is
created with no dictionaries, gdb will crash on the first line of
mdict_free:

  enum dict_type type = mdict->dictionaries[0]->vector->type;

So, this patch also adds the type to struct multidictionary, avoiding
this problem.  Note that this does not increase the structure size on
x86-64, because the new member fits into the padding.

Approved-By: Simon Marchi <simon.marchi@efficios.com>