a different lines->getRef() segfault

A new page was just coming in -- seems like the same sort of problem as
last time.

(gdb) bt
#0  0xb77bb424 in __kernel_vsyscall ()
#1  0xb72f9661 in *__GI_raise (sig=6)
    at ../nptl/sysdeps/unix/sysv/linux/raise.c:64
#2  0xb72fca92 in *__GI_abort () at abort.c:92
#3  0xb72f2878 in *__GI___assert_fail (
    assertion=0x8151afe "i >= 0 && this->num - i > 0", 
    file=0x8151aee "../lout/misc.hh", line=167, 
    function=0x8151ba0 "T* lout::misc::SimpleVector<T>::getRef(int) const [with T = dw::Textblock::Line]") at assert.c:81
#4  0x080a9040 in lout::misc::SimpleVector<dw::Textblock::Line>::getRef (
    this=0x953b618, i=-1) at ../lout/misc.hh:167
#5  0x080a2786 in dw::Textblock::sendSelectionEvent (this=0x916c610, 
    eventType=dw::core::SelectionState::BUTTON_RELEASE, event=0xbfbb8180)
    at textblock.cc:1093
#6  0x080a24c3 in dw::Textblock::buttonReleaseImpl (this=0x916c610, 
    event=0xbfbb8180) at textblock.cc:1008
#7  0x080d8910 in dw::core::Widget::buttonRelease (this=0x916c610, 
    event=0xbfbb8180) at widget.cc:1027
#8  0x080cd617 in dw::core::Layout::processMouseEvent (this=0x90fc058, 
    event=0xbfbb8180, type=dw::core::Layout::BUTTON_RELEASE) at layout.cc:1234
#9  0x080cd00e in dw::core::Layout::buttonEvent (this=0x90fc058, 
    type=dw::core::Layout::BUTTON_RELEASE, view=0x90f9f60, numPressed=1, 
    x=127, y=180, state=0, button=1) at layout.cc:1023
#10 0x080c46ae in dw::core::Layout::buttonRelease (this=0x90fc058, 
    view=0x90f9f60, numPressed=1, x=127, y=180, state=0, button=1)
    at layout.hh:345
#11 0x080c33f7 in dw::fltk::FltkViewBase::handle (this=0x90f9f60, event=2)
    at fltkviewbase.cc:319
#12 0x080c5e8a in dw::fltk::FltkViewport::handle (this=0x90f9f60, event=2)
    at fltkviewport.cc:331
#13 0x080ded22 in send(int, Fl_Widget*, Fl_Window*) ()
#14 0x080dfbaf in Fl::handle_(int, Fl_Window*) ()
#15 0x080dff0e in Fl::handle(int, Fl_Window*) ()
#16 0x08107e9d in fl_handle(_XEvent const&) ()
#17 0x08107fbc in do_queued_events() ()
#18 0x0810822a in fl_wait(double) ()
#19 0x080e08af in Fl::wait(double) ()
#20 0x080e099d in Fl::run() ()
#21 0x08052f7a in main (argc=1, argv=0xbfbb8854) at dillo.cc:588