Intermittent crash in macOS select operation

[jdm]

From a Servo test run:

 ▶ TIMEOUT [expected OK] /_mozilla/webgl/conformance-1.0.3/conformance/ogles/GL/log2/log2_009_to_012.html
  │ 
  │ VMware, Inc.
  │ softpipe
  │ 3.3 (Core Profile) Mesa 18.1.0-devel
  │ DESCRIPTION: WebGL GLSL conformance test: log2_009_to_012.html
  │ 
  │ test: log2_vec2_vert_xvary.test.html
  │ loading: log2_vec2_vert_xvary_ref.vert
  │ loading: ../default/default.frag
  │ loading: log2_vec2_vert_xvary.vert
  │ loading: ../default/default.frag
  │ loaded: ../default/default.frag
  │ loaded: ../default/default.frag
  │ loaded: log2_vec2_vert_xvary_ref.vert
  │ 
  │ loaded: log2_vec2_vert_xvary.vert
  │ 
  │ PASS images are the same
  │ PASS getError was expected value: NO_ERROR : there should be no errors
  │ 
  │ test: log2_vec2_vert_xvary01.test.html
  │ loading: log2_vec2_vert_xvary01_ref.vert
  │ loading: ../default/default.frag
  │ loading: log2_vec2_vert_xvary01.vert
  │ loading: ../default/default.frag
  │ loaded: log2_vec2_vert_xvary01_ref.vert
  │ loaded: log2_vec2_vert_xvary01.vert
  │ loaded: ../default/default.frag
  │ 
  │ loaded: ../default/default.frag
  │ 
  │ PASS images are the same
  │ PASS getError was expected value: NO_ERROR : there should be no errors
  │ 
  │ test: log2_vec3_vert_xvary.test.html
  │ loading: log2_vec3_vert_xvary_ref.vert
  │ loading: ../default/default.frag
  │ loading: log2_vec3_vert_xvary.vert
  │ loading: ../default/default.frag
  │ loaded: ../default/default.frag
  │ loaded: log2_vec3_vert_xvary_ref.vert
  │ 
  │ loaded: log2_vec3_vert_xvary.vert
  │ loaded: ../default/default.frag
  │ 
  │ PASS images are the same
  │ PASS getError was expected value: NO_ERROR : there should be no errors
  │ 
  │ test: log2_vec3_vert_xvary01.test.html
  │ loading: log2_vec3_vert_xvary01_ref.vert
  │ loading: ../default/default.frag
  │ loading: log2_vec3_vert_xvary01.vert
  │ loading: ../default/default.frag
  │ loaded: log2_vec3_vert_xvary01_ref.vert
  │ loaded: ../default/default.frag
  │ 
  │ 2018-07-09 11:40:32.314 servo[52311:15230532] Metadata.framework [Error]: couldn't get the client port
  │ loaded: log2_vec3_vert_xvary01.vert
  │ loaded: ../default/default.frag
  │ 
  │ Stack trace
  │ stack backtrace:
  │    0:        0x1084b944e - backtrace::backtrace::trace::h53c90f7e693c3e4a
  │    1:        0x1084b9efc - _$LT$backtrace..capture..Backtrace$u20$as$u20$core..default..Default$GT$::default::hf26626fd61b15e6b
  │    2:        0x1084b9f4d - backtrace::capture::Backtrace::new::he239b90a51ed8f1d
  │    3:        0x105c26526 - servo::install_crash_handler::handler::h0f99f069b49c2a95
  │    4:     0x7fff88a4af19 - _sigtramp
  │    5:        0x1083d6bb8 - ipc_channel::platform::macos::select::h84e2613789d2d719 (.llvm.14121967029294250274)
  │    6:        0x1083e239e - ipc_channel::ipc::IpcReceiverSet::select::h915cab13ee44775a
  │    7:        0x1083dd2ef - ipc_channel::router::Router::run::ha10e830819ba3a85
  │    8:        0x1083d7de6 - std::sys_common::backtrace::__rust_begin_short_backtrace::h872979c7e84759fc
  │    9:        0x1083d89d7 - std::panicking::try::do_call::h34b57e2e0e72baa2 (.llvm.334166917501917549)
  │   10:        0x1084eb67e - __rust_maybe_catch_panic
  │   11:        0x1083d883a - _$LT$F$u20$as$u20$alloc..boxed..FnBox$LT$A$GT$$GT$::call_box::h70e472b368be2ed3
  │   12:        0x1084c2b7b - std::sys::unix::thread::Thread::new::thread_start::hfdfbf4ce872c8486
  │   13:     0x7fff85b32059 - _pthread_body
  │   14:     0x7fff85b31fd6 - _pthread_start