libavutil: Use an intermediate variable in AV_COPY*U

If AV_RN and AV_WN are macros with multiple individual reads and
writes, the previous version of the AV_COPYU macro would fail if
the reads and writes overlap.

This should not be any less efficient in any case, given a
sensibly optimizing compiler.

Signed-off-by: Martin Storsjö <martin@martin.st>