rxvt sometimes segfaults on resizing operations, the
reasons seems to be that rxvt_scr_reset calls
rxvt_blank_screen_mem when adding rows.
However, rxvt_blank_screen_mem uses TemrWin.ncol, which
already contains the new (sometimes larger) value,
while the memory block is still smaller, resulting in
Setting TermWin.ncol to preV_ncol and restoring it in
the add rows branch fixes this.