multiswap_step() and multiswap_inv_step() both only require
six keys; in all current callers, these keys are part of
an array of twelve keys, yet in some of these callers the keys
given to these functions point to the second half of these
twelve keys, so that only six keys are available to these functions.
This led to -Wstringop-overread warnings when compiling with GCC 12.1.
Fix these by adapting the declaration of these functions.
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>