Bug 1145768 - Use the right begin() iterator to compare with. r=roc

Using the wrong begin() means that RFindLineContaining would hang
in the case the frame isn't on the overflow list.  We should always
find it there though, if it wasn't found on the principal list, so
the bug is harmless apart from the assertion.

layout/generic/nsBlockFrame.cpp

@@ -5086,7 +5086,8 @@ nsBlockFrame::AddFrames(nsFrameList& aFrameList, nsIFrame* aPrevSibling)
       if (overflowLines) {
         prevSibLine = overflowLines->mLines.end();
         prevSiblingIndex = -1;
-        found = nsLineBox::RFindLineContaining(aPrevSibling, lineList->begin(),
+        found = nsLineBox::RFindLineContaining(aPrevSibling,
+                                               overflowLines->mLines.begin(),
                                                prevSibLine,
                                                overflowLines->mFrames.LastChild(),
                                                &prevSiblingIndex);
@@ -5097,7 +5098,7 @@ nsBlockFrame::AddFrames(nsFrameList& aFrameList, nsIFrame* aPrevSibling)
       } else {
         // Note: defensive code! RFindLineContaining must not return
         // false in this case, so if it does...
-        NS_NOTREACHED("prev sibling not in line list");
+        MOZ_ASSERT_UNREACHABLE("prev sibling not in line list");
         aPrevSibling = nullptr;
         prevSibLine = lineList->end();
       }