if DrawBuffer set to GL_NONE and format is GL_DEPTH_COMPONENT, glDrawPixels will exit with segment fault.

The backtrace info:

Program received signal SIGSEGV, Segmentation fault.
[Switching to Thread -1211491728 (LWP 11027)]
0xb7aae7a0 in i915_do_emit_state (intel=0x8057c18) at i915_vtbl.c:359
359           OUT_RELOC(state->draw_region->buffer,
(gdb) bt
#0  0xb7aae7a0 in i915_do_emit_state (intel=0x8057c18) at i915_vtbl.c:359
#1  0xb7aaf58f in i915_emit_state (intel=0x8057c18) at i915_vtbl.c:466
#2  0xb7a9f46a in intelDrawPixels (ctx=0x8057c18, x=0, y=0, width=32,
    height=32, format=6402, type=5126, unpack=0x80638ac, pixels=0xbfae8160)
    at intel_pixel_draw.c:68
#3  0xb7be759c in _mesa_DrawPixels (width=32, height=32, format=6402,
    type=5126, pixels=0xbfae8160) at main/drawpix.c:186
#4  0xb7f58e52 in glDrawPixels (width=32, height=32, format=6402, type=5126,
    pixels=0xbfae8160) at ../../../src/mesa/glapi/glapitemp.h:1370
#5  0x08048a2d in test () at pxtrans-depth.c:30
#6  0x08048bd8 in display () at pxtrans-depth.c:58
#7  0xb7eed53f in processWindowWorkList (window=0x804eae8) at glut_event.c:1306
#8  0xb7eee012 in glutMainLoop () at glut_event.c:1353
#9  0x08048c54 in main (argc=1, argv=0xbfae9404) at pxtrans-depth.c:70

No matter if INTEL_NO_TTM is set, the bug exists.