Cannot reach dead statement "return NULL;"

Check for the NULLness of prop->name and prop->value instead of
name and value, which was checked earlier anyway. Decided against
using strdup due to curious memory allocation functions and the
rest of the xkb not using it either.
Signed-off-by: Erkki Seppälä <erkki.seppala@vincit.fi>
Reviewed-by: Alan Coopersmith <alan.coopersmith at oracle.com>
Backported-to-NX-by: Ulrich Sibiller <uli42@gmx.de>