Bug 1808182 - Use of uninitialised values originating from NS_GetComplexLineBreaks (in nsPangoBreaker.cpp). r=jfkthame.

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

intl/lwbrk/nsPangoBreaker.cpp

@@ -14,12 +14,15 @@ void NS_GetComplexLineBreaks(const char16_t* aText, uint32_t aLength,
                              uint8_t* aBreakBefore) {
   NS_ASSERTION(aText, "aText shouldn't be null");
 
-  memset(aBreakBefore, false, aLength * sizeof(uint8_t));
+  memset(aBreakBefore, uint8_t(false), aLength * sizeof(uint8_t));
 
   AutoTArray<PangoLogAttr, 2000> attrBuffer;
   // XXX(Bug 1631371) Check if this should use a fallible operation as it
   // pretended earlier.
   attrBuffer.AppendElements(aLength + 1);
+  // `PangoLogAttr` doesn't have a default constructor (it is a C struct), so
+  // we need to manually initialize the new elements.  See bug 1808182.
+  memset(attrBuffer.Elements(), 0, attrBuffer.Length() * sizeof(PangoLogAttr));
 
   NS_ConvertUTF16toUTF8 aUTF8(aText, aLength);