[layout] Initialize PerSpanData members mBaseline and mReflowInput.

We null-initialize all of the other pointer members in NewPerSpanData; we should
do the same for these ones, for consistency & robustness.

(In practice, the callers end up initializing these members before reading them
anyway, so it's been benign that we weren't initializing them. But better for
safety & futureproofing to have them reliably initialized.)

layout/generic/nsLineLayout.cpp

@@ -401,8 +401,10 @@ nsLineLayout::NewPerSpanData()
   psd->mFrame = nullptr;
   psd->mFirstFrame = nullptr;
   psd->mLastFrame = nullptr;
+  psd->mReflowInput = nullptr;
   psd->mContainsFloat = false;
   psd->mHasNonemptyContent = false;
+  psd->mBaseline = nullptr;
 
 #ifdef DEBUG
   outerLineLayout->mSpansAllocated++;