This avoids the risk of accidentally clobbering such variables outside
of the macro if the same variables are used there.
Signed-off-by: Martin Storsjö <martin@martin.st>

This fixes valgrind warnings about conditional jumps based on
uninitialized data (even though the uninitialized data only ever
was compared with a direct copy of the same uninitialized data).
Signed-off-by: Martin Storsjö <martin@martin.st>

The tests are inspired by similar tests for vp9 by
Ronald Bultje.
Signed-off-by: Martin Storsjö <martin@martin.st>