avoid overflowing pointers passed to memcpy

[qinheping]

After executing the for-loop in aws_array_list_mem_swap, the pointers item1 and item2 could be one past the boundaries. Therefore, both pointers could be outside the address space of the program and result in undefined behavior according to the C99 as follows (7.21.1 par 2):

Where an argument declared as size_t n specifies the length of the array for a function, n can have the value zero on a call to that function. Unless explicitly stated otherwise in the description of a particular function in this subclause, pointer arguments on such a call shall still have valid values, as described in 7.1.4..

Issue: diffblue/cbmc#6472.

Description of changes: stop calling memcpy if the remainder is zero.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[jeking3]

This seems reasonable to me given the description. It is low risk, high reward.