segfault in mir::compositor::GLProgramFamily::Shader::init()

This is intermittent (so I'm not sure if there are other ways to reproduce) but Initial experiments suggest this comes of running mir_demo_server with the option "--display-config sidebyside" (and two monitors).

Here's some gdb of a core:

(gdb) info threads
  Id Target Id Frame
  8 Thread 0x7f3e8effd700 (LWP 4942) 0x00007f3e9cbb8e1d in close () at ../sysdeps/unix/syscall-template.S:81
  7 Thread 0x7f3e8f7fe700 (LWP 4941) 0x00007f3e9c3b1283 in select () at ../sysdeps/unix/syscall-template.S:81
  6 Thread 0x7f3e8e7fc700 (LWP 4943) 0x00007f3e9c3b9ee3 in epoll_wait () at ../sysdeps/unix/syscall-template.S:81
  5 Thread 0x7f3e95b6a700 (LWP 4937) pthread_cond_wait@@GLIBC_2.3.2 ()
    at ../nptl/sysdeps/unix/sysv/linux/x86_64/pthread_cond_wait.S:185
  4 Thread 0x7f3e95369700 (LWP 4938) 0x00007f3e9c3b9ee3 in epoll_wait () at ../sysdeps/unix/syscall-template.S:81
  3 Thread 0x7f3e94b68700 (LWP 4939) pthread_cond_wait@@GLIBC_2.3.2 ()
    at ../nptl/sysdeps/unix/sysv/linux/x86_64/pthread_cond_wait.S:185
  2 Thread 0x7f3e9dd22780 (LWP 4935) 0x00007f3e9c3af45d in poll () at ../sysdeps/unix/syscall-template.S:81
* 1 Thread 0x7f3e8ffff700 (LWP 4940) 0x00007f3e9683bc2b in ?? () from /usr/lib/x86_64-linux-gnu/dri/i965_dri.so
(gdb) bt
#0 0x00007f3e9683bc2b in ?? () from /usr/lib/x86_64-linux-gnu/dri/i965_dri.so
#1 0x00007f3e9683bc89 in ?? () from /usr/lib/x86_64-linux-gnu/dri/i965_dri.so
#2 0x00007f3e9683c1f5 in ?? () from /usr/lib/x86_64-linux-gnu/dri/i965_dri.so
#3 0x00007f3e96816ec7 in ?? () from /usr/lib/x86_64-linux-gnu/dri/i965_dri.so
#4 0x00007f3e9686169f in ?? () from /usr/lib/x86_64-linux-gnu/dri/i965_dri.so
#5 0x00007f3e966fbfcf in ?? () from /usr/lib/x86_64-linux-gnu/dri/i965_dri.so
#6 0x00007f3e9d60d542 in mir::compositor::GLProgramFamily::Shader::init (this=0x7f3e80002550, type=35632,
    src=0x7f3e9d7a09a8 "precision mediump float;\nuniform sampler2D tex;\nuniform float alpha;\nvarying vec2 v_texcoord;\nvoid main() {\n vec4 frag = texture2D(tex, v_texcoord);\n gl_FragColor = alpha*frag;\n}\n")
    at /home/alan/display_server/mir3/src/server/compositor/gl_program_family.cpp:31
#7 0x00007f3e9d60d9fa in mir::compositor::GLProgramFamily::add_program (this=0x7f3e800009f0,
    vshader_src=0x7f3e9d7a0808 "attribute vec3 position;\nattribute vec2 texcoord;\nuniform mat4 screen_to_gl_coords;\nuniform mat4 display_transform;\nuniform mat4 transform;\nuniform vec2 centre;\nvarying vec2 v_texcoord;\nvoid main() {\n"...,
    fshader_src=0x7f3e9d7a09a8 "precision mediump float;\nuniform sampler2D tex;\nuniform float alpha;\nvarying vec2 v_texcoord;\nvoid main() {\n vec4 frag = texture2D(tex, v_texcoord);\n gl_FragColor = alpha*frag;\n}\n")
    at /home/alan/display_server/mir3/src/server/compositor/gl_program_family.cpp:80
#8 0x00007f3e9d6090e5 in mir::compositor::GLRenderer::GLRenderer(std::unique_ptr<mir::graphics::GLTextureCache, std::default_delete<mir::graphics::GLTextureCache> >&&, mir::geometry::Rectangle const&, mir::compositor::DestinationAlpha) (
    this=0x7f3e800009d0, texture_cache=<unknown type in bin/../lib/libmirserver.so.29, CU 0x4bf826, DIE 0x4d008e>,
    display_area=..., dest_alpha=mir::compositor::DestinationAlpha::opaque)
    at /home/alan/display_server/mir3/src/server/compositor/gl_renderer.cpp:102
#9 0x00007f3e9d60cb45 in mir::compositor::GLRendererFactory::create_renderer_for (this=0x26c7cb0, rect=...,
    dest_alpha=mir::compositor::DestinationAlpha::opaque)
    at /home/alan/display_server/mir3/src/server/compositor/gl_renderer_factory.cpp:35
#10 0x00007f3e9d603764 in mir::compositor::DefaultDisplayBufferCompositorFactory::create_compositor_for (this=0x2585d00,
    display_buffer=...)
    at /home/alan/display_server/mir3/src/server/compositor/default_display_buffer_compositor_factory.cpp:44