Bug 1361580 - Initialize nsContinuingTextFrame::mPrevContinuation. r=…

…dholbert

In `nsContinuingTextFrame::Init()` [1], we always call `SetPrevInFlow()` to
initialize `mPrevContinuation`, so we are fine. Still, it is better to
initialize the pointer properly before `Init()`.

[1] https://searchfox.org/mozilla-central/rev/d7f837add602d270f2b2958b3ab5206dc85965c0/layout/generic/nsTextFrame.cpp#4230

Differential Revision: https://phabricator.services.mozilla.com/D194407

layout/generic/nsTextFrame.cpp

@@ -4215,7 +4215,7 @@ class nsContinuingTextFrame final : public nsTextFrame {
                                  nsPresContext* aPresContext)
       : nsTextFrame(aStyle, aPresContext, kClassID) {}
 
-  nsTextFrame* mPrevContinuation;
+  nsTextFrame* mPrevContinuation = nullptr;
   nsTextFrame* mFirstContinuation = nullptr;
 };