Fix buffer corruption issue of last_over_time with native histograms

[krajorama]

The test defined by

# Add histogram to test sum(last_over_time) regression
load 5m
    incr_sum_histogram{number="1"} {{schema:0 sum:0 count:0 buckets:[1]}}+{{schema:0 sum:1 count:1 buckets:[1]}}x10
    incr_sum_histogram{number="2"} {{schema:0 sum:0 count:0 buckets:[1]}}+{{schema:0 sum:2 count:1 buckets:[1]}}x10

eval instant at 50m histogram_sum(sum(incr_sum_histogram))
    {} 30

eval instant at 50m histogram_sum(sum(last_over_time(incr_sum_histogram[5m])))
    {} 30

Should pass, but results in

error in eval histogram_sum(sum(last_over_time(incr_sum_histogram[1m]))) (line 237): expected 30 for {} but got 40

Works before #13340

[krajorama]

Note: if I change the line
from

ss.Histograms = append(ss.Histograms, HPoint{H: outVec[0].H, T: ts})

to

ss.Histograms = append(ss.Histograms, HPoint{H: outVec[0].H.Copy(), T: ts})

The problem goes away, but I don't understand yet why.

[krajorama]

proper fix in related PR