This code obviously should also use the cache_fields flag variable,
like the code for reading the lowres data in the same function.
This is because fluid_fields actually represents the old state before
smoke was reallocated to match cache_fields read from the file, and if
it has some fields enabled that aren't allocated any more, it crashes.
This also fixes a reverse glitch: when a file was loaded with
the current frame in the middle of a baked smoke+fire simulation,
smoke appeared immediately, but the fire didn't until the frame
was changed. The reason is the same: after file load no fields
are initially allocated and thus fluid_fields is 0.
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