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N3322 makes NULL + 0 well-defined in C, matching the C++ semantics. Adjust the pointer-overflow sanitizer to no longer report NULL + 0 as a pointer overflow in any language mode. NULL + nonzero will of course continue to be reported. As N3322 is part of https://www.open-std.org/jtc1/sc22/wg14/www/previous.html, and we never performed any optimizations based on NULL + 0 being undefined in the first place, I'm applying this change to all C versions.