multi-host qualified binary expression fails due to num_samples mismatch

[goodwinos]

$ pmseries 'kernel.all.load{hostname:"node80"}+kernel.all.load{hostname:"node81"}'
b4ba82162ff2a19eaaffab34db8d933dc0f94885
$ curl -Gs 'http://localhost:44322/series/values?series=b4ba82162ff2a19eaaffab34db8d933dc0f94885&count=1' | pmjson

Error:

pmseries_log (level=PMLOG_ERROR,
    message=0x5555556d7275 "Number of samples of two metrics are not identical, f640ce5806069843fea87132f43fd26423c30487 has 22 but 726a325c4c1ba4339ecffcdebd240f441ea77848 has 4367\n", arg=0x555555ebd130) at series.c:634

This will be a common scenario because the pmloggers for the two sources will likely have been running for different timespans and so there will basically always be a different number of samples.

Fix should take the smaller number of samples (22 in this case) and proceed with the expression calculation.