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 (closed).
(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 email@example.com