set_var_thresh_from_histogram: prevent negative variance

For 8-bit the subtrahend is small enough to fit into uint32_t.

For 10/12-bit apply:
63a37d16f Prevent negative variance

previously:
47b9a0912 Resolve -Wshorten-64-to-32 in highbd variance.
c0241664a Resolve -Wshorten-64-to-32 in variance.

Change-Id: I181c85f0b9a03da37c2e8b89482d48aa3dbc0aee
1 file changed