memory corruption fix for distributed data parallel version before SyncUpGlobalBestSplit

[imatiach-msft]

memory corruption was found using valgrind

see relevant issue for debugging info:
#2981

[StrikerRUS]

Close-reopen to retrigger Appveyor.

[imatiach-msft]

Close-reopen to retrigger travis URL checker

[imatiach-msft]

the PR previously failed on a Mac build that complained about different types passed to std::max, I added a fix for it (cast) that seems to have resolved the build issue

[imatiach-msft]

@guolinke might you be able to take another look at this PR? thank you!

[imatiach-msft]

close reopen for CI (URL checker failed)

URL        `https://en.wikipedia.org/wiki/Gamma_distribution#Occurrence_and_applications'

Name       `gamma-distributed'

Parent URL file:///home/travis/build/microsoft/LightGBM/docs/_build/html/Parameters.html, line 239, col 220

Real URL   https://en.wikipedia.org/wiki/Gamma_distribution#Occurrence_and_applications

Check time 3.469 seconds

D/L time   1.042 seconds

Size       345.98KB

Warning    [None] Anchor `Occurrence_and_applications' not found.

           Available anchors: `coll-download-as-rl', `footer',

           `footer-copyrightico', `footer-icons', `footer-info',

           `footer-info-copyright', `footer-info-lastmod',

           `footer-places', `footer-places-about',

           `footer-places-contact',

           `footer-places-cookiestatement',

           `footer-places-developers',

           `footer-places-disclaimer',

           `footer-places-mobileview', `footer-places-privacy',

           `footer-places-statslink', `footer-poweredbyico',

           `n-aboutsite', `n-contactpage', `n-portal',

           `n-recentchanges', `p-coll-print_export',

           `p-coll-print_export-label', `p-lang', `p-lang-label',

           `p-tb', `p-tb-label', `p-wikibase-otherprojects',

           `p-wikibase-otherprojects-label', `t-cite', `t-info',

           `t-permalink', `t-print', `t-recentchangeslinked',

           `t-specialpages', `t-upload', `t-whatlinkshere',

           `t-wikibase'.

[github-actions]

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