[patch] libstdc++/66327 don't pass null pointers to memcmp

[Jonathan Wakely]

std::equal((int*)0, (int*)0, p) and std::equal(p, p, (int*)0) are
valid for any input iterator p, and must not pass a null pointer to
memcpy.

Similarly, std::lexicographical_compare((int*)0, (int*)0, p, q) and
std::lexicographical_compare(p, q, (int*)0, (int*)0) are valid for any
input iterators p and q, and must not pass a null pointer to memcpy.

This is a rather brute force fix, but if noone has any better ideas
I'll commit this to the trunk and 4.9 and 5 branches tomorrow. (I
think it should go on the branches, because 4.9 is known to optimise
away null pointer checks after invalid calls to memcmp like this).

I am not adding tests for these as we have no way to use ubsan in the
testsuite at present. I plan to make that possible, and will go back
and add tests using -fsanitize=undefined for all the recent fixes I've
made for ubsan errors.

Tested powerpc64le-linux.
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