Even though the elements kind is FAST_DOUBLE_ELEMENTS, if length is zero
the isolate's empty_fixed_array is used. It's illegal to cast this to
FixedDoubleArray, so we avoid the cast.

BUG=chromium:686479

Review-Url: https://codereview.chromium.org/2665313003
Cr-Commit-Position: refs/heads/master@{#42867}