merge_into: accommodate python 3

merge_into: accommodate python 3

Switch to Py_buffers to accommodate python 3, use malloc/calloc to
avoid potentially involving the GIL, and check allocation failures.

Thanks to Johannes Berg for pointing out a potential overflow on the C
side -- fixed by adjusting checked_malloc() to accept the same
arguments as checked_calloc().

Signed-off-by: Rob Browning <rlb@defaultvalue.org>
Tested-by: Rob Browning <rlb@defaultvalue.org>