Backport r65182. This change modified from using the unsigned max value
to the signed max value similar to 2.5 and trunk.
Issue #2620: Overflow checking when allocating or reallocating memory
was not always being done properly in some python types and extension
modules. PyMem_MALLOC, PyMem_REALLOC, PyMem_NEW and PyMem_RESIZE have
all been updated to perform better checks and places in the code that
would previously leak memory on the error path when such an allocation
failed have been fixed.