util: Don't access members of NULL pointers

util: Don't access members of NULL pointers

Should be harmless, but UBSAN complains about it and fills the logs with
noise.

../src/gallium/auxiliary/util/u_inlines.h:110:8: runtime error: member access within null pointer of type 'struct pipe_surface'"}
    #0 0xaaccf186 in pipe_surface_reference ../src/gallium/auxiliary/util/u_inlines.h:110"}
    #1 0xaaccf186 in util_copy_framebuffer_state ../src/gallium/auxiliary/util/u_framebuffer.c:105"}
    #2 0xaabfb60e in cso_set_framebuffer ../src/gallium/auxiliary/cso_cache/cso_context.c:723"}
    #3 0xaae195ce in st_update_framebuffer_state ../src/mesa/state_tracker/st_atom_framebuffer.c:207"}
    #4 0xaae12316 in st_validate_state ../src/mesa/state_tracker/st_atom.c:261"}
    #5 0xaae31302 in st_Clear ../src/mesa/state_tracker/st_cb_clear.c:438"}
    #6 0x4c3d0e in deqp::gles2::TestCaseWrapper::iterate(tcu::TestCase*) (/deqp/modules/gles2/deqp-gles2+0x2ad0e)"}
    #7 0x828cf2 in tcu::TestSessionExecutor::iterateTestCase(tcu::TestCase*) (/deqp/modules/gles2/deqp-gles2+0x38fcf2)"}
    #8 0x8295f0 in tcu::TestSessionExecutor::iterate() (/deqp/modules/gles2/deqp-gles2+0x3905f0)"}
    #9 0x810aac in tcu::App::iterate() (/deqp/modules/gles2/deqp-gles2+0x377aac)"}
    #10 0x4c1d4c in main (/deqp/modules/gles2/deqp-gles2+0x28d4c)"}
    #11 0xb64b6aa8 in __libc_start_main (/lib/arm-linux-gnueabihf/libc.so.6+0x1aaa8)"}

Signed-off-by: Tomeu Vizoso <tomeu.vizoso@collabora.com>
Reviewed-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>