Issue 27570: Avoid memcpy(. . ., NULL, 0) etc calls

This patch fixes errors reported by GCC’s undefined behaviour sanitizer about calling functions with a null pointer:

./configure CC="gcc -fsanitize=undefined"

Using Issue 22605 as a precedent, I propose to avoid calling memcpy() and memmove() for zero-length copies when there may be a null pointer.

V2 patch adds another fix, uncovered by recent datetime tests:

>>> a = array("B")
>>> a[:] = a
/media/disk/home/proj/python/cpython/Modules/arraymodule.c:748:5: runtime error: null pointer passed as argument 1, which is declared to never be null
/media/disk/home/proj/python/cpython/Modules/arraymodule.c:748:5: runtime error: null pointer passed as argument 2, which is declared to never be null

Patch 3 fixes some more cases I found in the array module:

>>> a + a
Modules/arraymodule.c:809:5: runtime error: null pointer passed as argument 1, which is declared to never be null
Modules/arraymodule.c:809:5: runtime error: null pointer passed as argument 2, which is declared to never be null
Modules/arraymodule.c:810:5: runtime error: null pointer passed as argument 1, which is declared to never be null
Modules/arraymodule.c:810:5: runtime error: null pointer passed as argument 2, which is declared to never be null
array('B')
>>> a * 3
Modules/arraymodule.c:840:9: runtime error: null pointer passed as argument 1, which is declared to never be null
Modules/arraymodule.c:840:9: runtime error: null pointer passed as argument 2, which is declared to never be null
array('B')
>>> a += a
Modules/arraymodule.c:952:5: runtime error: null pointer passed as argument 1, which is declared to never be null
Modules/arraymodule.c:952:5: runtime error: null pointer passed as argument 2, which is declared to never be null

I wondered if there is a good argument for fixing these, or if it is only a theoretical problem. Apparently GCC can do optimizations about null pointer tests: <https://gcc.gnu.org/gcc-4.9/porting_to.html>. I don’t think any of the cases I found are instances of this problem, but I think fixing them helps keep the UB sanitizer output clean, so any errors causing practical behaviour problems will be easier to find.

Looks like revisions 5f3f6f1fb73a and ec537f9f468f may have fixed the listobject cases.

Also 66feda02f2a5 looks relevant. Benjamin, maybe you are interested in other bits of my patches :)

Sorry I missed this. The changes which I didn't already make look good. :)

New changeset e231dcad3a9b by Martin Panter in branch '3.5':
Issue #27570: Avoid zero-length memcpy() calls with null source pointers
https://hg.python.org/cpython/rev/e231dcad3a9b

New changeset 2d0fb659372c by Martin Panter in branch 'default':
Issue #27570: Merge null pointer fixes from 3.5
https://hg.python.org/cpython/rev/2d0fb659372c

New changeset d465da1e5902 by Martin Panter in branch '2.7':
Issue #27570: Avoid zero-length memcpy() calls with null source pointers
https://hg.python.org/cpython/rev/d465da1e5902

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