handle the max theoretical triangle clip results without stack crashing.

Each triangle might be clipped into two for all six planes, resulting in
64 triangles.  Fixes this bug:

Using host libthread_db library "/lib/i386-linux-gnu/libthread_db.so.1".
Core was generated by `./snis_limited_client'.
Program terminated with signal 11, Segmentation fault.
#0  0x3f7ffff0 in ?? ()
(gdb) bt
#0  0x3f7ffff0 in ?? ()
#1  0x08086409 in clip_triangle (triangles=0xbf8a05e0, vs_out0=0x9221fc0, vs_out1=0x9221908,
    vs_out2=0x9221840) at entity.c:1535
#2  0x0808357e in software_render_entity_triangles (w=0x8ff2840, gc=0x9056ea0, cx=0x99c1e80,
    e=0x9a23df0) at entity.c:509
#3  0x08083c57 in render_entity (w=0x8ff2840, gc=0x9056ea0, cx=0x99c1e80, e=0x9a23df0) at entity.c:605
#4  0x080852e0 in render_entities (w=0x8ff2840, gc=0x9056ea0, cx=0x99c1e80) at entity.c:1094
#5  0x08062a74 in show_mainscreen (w=0x8ff2840) at snis_limited_client.c:4910
#6  0x080797f0 in main_da_expose (w=0x8ff2840, event=0xbf8a0ee8, p=0x0) at snis_limited_client.c:10887
#7  0xb73b28a2 in ?? () from /usr/lib/i386-linux-gnu/libgtk-x11-2.0.so.0
#8  0xb7188484 in g_closure_invoke () from /usr/lib/i386-linux-gnu/libgobject-2.0.so.0
#9  0xb719a0d9 in ?? () from /usr/lib/i386-linux-gnu/libgobject-2.0.so.0
#10 0xb71a1ff5 in g_signal_emit_valist () from /usr/lib/i386-linux-gnu/libgobject-2.0.so.0
#11 0xb71a2453 in g_signal_emit () from /usr/lib/i386-linux-gnu/libgobject-2.0.so.0
#12 0xb74ec413 in ?? () from /usr/lib/i386-linux-gnu/libgtk-x11-2.0.so.0
#13 0xb73b10ff in gtk_main_do_event () from /usr/lib/i386-linux-gnu/libgtk-x11-2.0.so.0
#14 0xb72082bf in ?? () from /usr/lib/i386-linux-gnu/libgdk-x11-2.0.so.0
#15 0xb7208262 in ?? () from /usr/lib/i386-linux-gnu/libgdk-x11-2.0.so.0
#16 0xb723acd3 in ?? () from /usr/lib/i386-linux-gnu/libgdk-x11-2.0.so.0
#17 0xb7202dc4 in ?? () from /usr/lib/i386-linux-gnu/libgdk-x11-2.0.so.0
#18 0xb720504f in gdk_window_process_all_updates () from /usr/lib/i386-linux-gnu/libgdk-x11-2.0.so.0
#19 0xb72050c8 in ?? () from /usr/lib/i386-linux-gnu/libgdk-x11-2.0.so.0
#20 0xb71dff88 in ?? () from /usr/lib/i386-linux-gnu/libgdk-x11-2.0.so.0
---Type <return> to continue, or q <return> to quit---
#21 0xb70c6850 in ?? () from /lib/i386-linux-gnu/libglib-2.0.so.0
#22 0xb70c8d86 in g_main_context_dispatch () from /lib/i386-linux-gnu/libglib-2.0.so.0
#23 0xb70c9125 in ?? () from /lib/i386-linux-gnu/libglib-2.0.so.0
#24 0xb70c956b in g_main_loop_run () from /lib/i386-linux-gnu/libglib-2.0.so.0
#25 0xb73afb8f in gtk_main () from /usr/lib/i386-linux-gnu/libgtk-x11-2.0.so.0
#26 0x0807bf1d in main (argc=1, argv=0xbf8a1364) at snis_limited_client.c:11811

To reproduce the bug (without this commit, obviously) park the ship inside a
starbase and let the starbase spin around with the main view up.
Usually crashes pretty quickly.