emcmodule.cc: fail gracefully for linuxcnc.ini()

avoid segfault in Ini_dealloc()

before patch:
      $ python -c 'import linuxcnc; x=linuxcnc.ini()'
      Segmentation fault (core dumped)

after:
      $ python -c 'import linuxcnc; x=linuxcnc.ini()'
      Traceback (most recent call last):
        File "<string>", line 1, in <module>
      TypeError: function takes exactly 1 argument (0 given)

similar behavior for  x=linuxcnc.ini(None)
Don't leak file descriptors or memory when abused

It's possible to call Ini_init multiple times, don't leak a new IniFile
or a file descriptor each time.

In Ini_dealloc it's still necessary to PyObjectDel self; it's defined as
OK (a no-op) to delete a NULL pointer, so the delete statement doesn't
need to be guarded by the test.