Using uninitialized value "p->modifiers"

Small fix by using Xcalloc instead of Xmalloc
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>