Backport of r61180:
Added checks for integer overflows, contributed by Google. Some are
only available if asserts are left in the code, in cases where they
can't be triggered from Python code.

• Include/pymem.h[diff]
• Include/pyport.h[diff]
• Misc/NEWS[diff]
• Modules/_csv.c[diff]
• Modules/arraymodule.c[diff]
• Modules/audioop.c[diff]
• Modules/binascii.c[diff]
• Modules/cPickle.c[diff]
• Modules/cStringIO.c[diff]
• Modules/datetimemodule.c[diff]
• Modules/rgbimgmodule.c[diff]
• Modules/stropmodule.c[diff]
• Objects/bufferobject.c[diff]
• Objects/listobject.c[diff]
• Parser/node.c[diff]
• Python/bltinmodule.c[diff]