cmd-key-happy: assert on the Lua stack depth
cmd-key-happy: assert on the Lua stack depth

Should always be 0. This commit adds a missing pop() to cases where
there was an error message from a call to swap_keys().

This commit will now always raise an assert message, irrespective of the
value of NDEBUG.

andymcd/cmd-key-happy,andymcd/cmd-key-happy,frobware/cmd-key-happy,andymcd/cmd-key-happy,frobware/cmd-key-happy,frobware/cmd-key-happy,frobware/cmd-key-happy,andymcd/cmd-key-happy,frobware/cmd-key-happy,andymcd/cmd-key-happy