XKBMAlloc: use the correct first-key variable when extending a key range

When extending the range of changed virtual-modifier keys, one should
pass the two variables relevant to that range.

These two mistakes were found by grepping for 'modmap_key' and then
looking for asymmetries between the handling of the plain modmap thing
and the virtual modmap thing.  Inspired by issue #76.

(I suspect these mistakes were at the base of some intractable bugs
reported against xkeyboard-config where virtual modifiers refused
to work and had to be worked around by making other changes too.)

Signed-off-by: Benno Schulenberg <bensberg@telfort.nl>
4 jobs for vmodmap-range in 12 minutes and 16 seconds (queued for 2 seconds)