FD leak with mmap - fix and reproducer

[bartku]

File descriptor leak during using of mmap for CDB creation has been observed.

Leak does not occur if CDB.make(fd, mmap=False) is used. If mmap is set to either True or None then CDB does not close the file.

Leak can be reproduced using script in tools/fd_leak.py.

Root cause
Function cdb32_mmap() uses mmap through PyObject_Call(). Returned Py_Object is passed to PyObject_GetBuffer(). Object is stored in view Py_Buffer but never released.

[andre-malo]

Thank you!

The fix looks correct and I'll incorporate it. While checking out the python code I realized this should have failed on mmap.close(). Which made me look again and I realized I'm also not closing the mmap (I'm simply destroying it). The practical effect is probably zero, but it will open up the possibility of a nice and short regression test, instead of running the fullblown fd counter.