Intermittent crash in hashtable code

[jdm]

From an earlier run of #19829, this crash was observed:

  ▶ TIMEOUT [expected PASS] /css/CSS2/box-display/containing-block-009.xht
  │ 
  │ VMware, Inc.
  │ softpipe
  │ 3.3 (Core Profile) Mesa 17.3.0-devel
  │ 2018-01-22 10:30:57.066 servo[81885:285780639] Metadata.framework [Error]: couldn't get the client port
  │ Stack trace for thread "WRRenderBackend#0"
  │ stack backtrace:
  │    0:        0x10f8f3aee - backtrace::backtrace::trace::h1572cbde15e97645
  │    1:        0x10f8f42dc - backtrace::capture::Backtrace::new::h0ca27cd1e659fd9b
  │    2:        0x10d3219a0 - servo::install_crash_handler::handler::hef6d72ec40496654
  │    3:     0x7fff8f88ff19 - _sigtramp
  │    4:        0x10ef1a4c7 - _$LT$std..collections..hash..map..HashMap$LT$K$C$$u20$V$C$$u20$S$GT$$GT$::contains_key::h6f7b7a7f90f60aa7
  │    5:        0x10eed3f15 - webrender::glyph_rasterizer::_$LT$impl$u20$webrender..platform..macos..font..FontContext$GT$::add_font::h855f3dfb504b8fe4
  │    6:        0x10ef64457 - webrender::glyph_rasterizer::GlyphRasterizer::add_font::h8f362be6e920367e
  │    7:        0x10eefea16 - webrender::resource_cache::ResourceCache::add_font_template::hd9fa88ab61ef0758
  │    8:        0x10eefd306 - webrender::resource_cache::ResourceCache::update_resources::h4a4d139987a02082
  │    9:        0x10ef4d9c9 - webrender::render_backend::RenderBackend::run::h3fba174366c9d443
  │   10:        0x10ee879e1 - std::sys_common::backtrace::__rust_begin_short_backtrace::h3370e1c89a690f63
  │   11:        0x10eeb9b1d - std::panicking::try::do_call::h939a1adbd2b681c9 (.llvm.BA14D8D4)
  │   12:        0x10f923fae - __rust_maybe_catch_panic
  │   13:        0x10ef59f93 - _$LT$F$u20$as$u20$alloc..boxed..FnBox$LT$A$GT$$GT$::call_box::heff2fe36abcdc229
  │   14:        0x10f90cd67 - std::sys_common::thread::start_thread::h1ef8cf6b973abe77
  │   15:        0x10f9100e8 - std::sys::unix::thread::Thread::new::thread_start::h88d15e6243c319fe
  │   16:     0x7fff91ccf059 - _pthread_body
  │   17:     0x7fff91ccefd6 - _pthread_start

The fact that we're segfaulting in HashMap::contains_key is deeply scary to me.