Fix a nasty endcase reported by Armin Rigo in SF bug 618623:

Fix a nasty endcase reported by Armin Rigo in SF bug 618623:
'%2147483647d' % -123 segfaults.  This was because an integer overflow
in a comparison caused the string resize to be skipped.  After fixing
the overflow, this could call _PyString_Resize() with a negative size,
so I (1) test for that and raise MemoryError instead; (2) also added a
test for negative newsize to _PyString_Resize(), raising SystemError
as for all bad arguments.

An identical bug existed in unicodeobject.c, of course.

Will backport to 2.2.2.