Issue 6271: mmap: don't close file description if fd=-1

Hi, Valgrind just told me that Python calls close(-1) on
my_mmap_object.close() for memory mappings. That's because a memory
mapping has no (related) file descriptor.

Using attached warn.py, you can see the warning using strace:

$ strace -e close python warn.py 2>&1|grep -A1 12345
close(12345)                            = -1 EBADF (Bad file descriptor)
close(4294967295)                       = -1 EBADF (Bad file descriptor)
close(12345)                            = -1 EBADF (Bad file descriptor)

where close(4294967295) means close(-1).

Attached patch fixes this warning.