doh: fix undefined behaviour in swap #4350
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The undefined behaviour is annoying when running fuzzing with
sanitizers. The codegen is the same, but the meaning is now
not up for dispute. See https://cppinsights.io/s/516a2ff4
By incrementing the pointer first, both gcc and clang recognize this
as a bswap and optimizes it to a single instruction.
See https://godbolt.org/z/994Zpx