[processing] fix intersection and union tools

[alexbruy]

Description

This is backport of master fixes to 2.18 branch

Checklist

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[alexbruy]

@gioman can you test these fixes?

[alexbruy]

Travis failure on master expected and not related, as this PR targeted to 2.18 branch

[nyalldawson]

Travis failure on master expected and not related, as this PR targeted to 2.18 branch

It's not failing on master, it's failing on the 2.18 build under qt5. So it must be something related to these changes.

[nyalldawson]

It's not failing on master, it's failing on the 2.18 build under qt5. So it must be something related to these changes.

Actually it IS an unrelated test failure ;)

[gioman]

@gioman can you test these fixes?

will do asap!

[gioman]

see
https://issues.qgis.org/issues/17131
and
https://issues.qgis.org/issues/17132

[Gustry]

See https://issues.qgis.org/issues/14846 too
@wonder-sk was fixing this ticket in the 2.18 branch too (AFAIK)

[gioman]

@Gustry I added a new comment https://issues.qgis.org/issues/17131#note-2 that is about the dataset provided by the issuer of https://issues.qgis.org/issues/14846 and upatched processing in 2.18.12

Master and the patched processing for 2.18.12 that included backports from master gives results that are worst than the unpatched processing in 2.18.12. In this case with very simple datasets the results are sometimes correct, in other cases are still wrong but "look" better than on master.