Variable "map" goes out of scope

Release modifiermap before returning. Reordered code to call
XGetModifierMapping after the first return from the function.
Signed-off-by: Erkki Seppälä <erkki.seppala@vincit.fi>
Reviewed-by: Dirk Wallenstein <halsmit@t-online.de>
Backported-to-NX-by: Ulrich Sibiller <uli42@gmx.de>