NS_GetComplexLineBreaks (in nsPangoBreaker.cpp) adds elements of type
`PangoLogAttr` to an array with `attrBuffer.AppendElements(aLength + 1);`.
However, `PangoLogAttr` doesn't have a default constructor, so those elements
are uninitialised, and that eventually leaks back to the the caller,
`ComplexBreaker::GetBreaks` and are used in a couple of different places after
that. This patch fixes that by manually zeroing out the new area.
Differential Revision: https://phabricator.services.mozilla.com/D165846
d71b143fb3