re_intuit_start(): fix byte/char calculation err

re_intuit_start(): fix byte/char calculation err

On failure to find stclass in float range, it currently does:

    rx_origin = check_at - start_shift;

which should instead be

    rx_origin = HOP3c(check_at, - start_shift, strbeg);

The second added test is an example of when this causes a false negative.
(The first test is just the analogue without a UTF8 string, so it already
passes).

Since we actually already calculate this value earlier as part of
determining endpos in the float case, assign the result of that calc to a
local var, then just reuse it.