doh: fix undefined behaviour in swap

[pauldreik]

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

[bagder]

There are certainly several ways to fix the undefined behavior (even if most compilers seem to be fine with it), but if this has the benefit that it generates better code I'm all for it!

[bagder]

Thanks!