Discrete axis fallback fails if glyph isn't interpolatable

[justvanrossum]

If a glyph doesn't have non-default sources for a discrete axis, we normally fall back to the default. For example, /A exists for ital=0, but does not exist for ital=1, then we show ital=0 in gray as a fallback. This mechanism has a bug when the ital=0 glyph isn't interpolatable.

utils.b7c094fd.js:164 Uncaught (in promise) TypeError: iterable is not iterable
    at enumerate (utils.b7c094fd.js:164:22)
    at enumerate.next (<anonymous>)
    at findNearestLocationIndex (discrete-variation-m….b7c094fd.js:232:25)
    at BrokenVariationModel.interpolateFromDeltas (discrete-variation-m….b7c094fd.js:195:19)
    at DiscreteVariationModel.interpolateFromDeltas (discrete-variation-m….b7c094fd.js:119:28)
    at VariableGlyphController.instantiateSync (glyph-controller.b7c094fd.js:397:43)
    at VariableGlyphController.instantiate (glyph-controller.b7c094fd.js:393:17)
    at async VariableGlyphController.instantiateController (glyph-controller.b7c094fd.js:431:34)
    at async FontController._getGlyphInstance (font-controller.b7c094fd.js:579:32)
    at async FontController.getGlyphInstance (font-controller.b7c094fd.js:570:12)