Auto merge of #1664 - lsalzman:instance-key-fail, r=glennw
Auto merge of #1664 - lsalzman:instance-key-fail, r=glennw

don't panic when an invalid font instance key is supplied

This is a small modification to my previous FontInstanceKey to allow invalid font instance keys to work around the issue described in https://bugzilla.mozilla.org/show_bug.cgi?id=1396056

There are somewhat legitimate situations where this can occur, so we don't want to panic.

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