Fix incorrect tinderbox popups with fastback (bug 292933). Make sure mLSHE is nulled out when we finish fastback navigation. r+sr=darin, a=brendan.

docshell/base/nsDocShell.cpp

@@ -5188,8 +5188,13 @@ nsDocShell::RestorePresentation(nsISHEntry *aSHEntry, PRBool aSavePresentation,
         nsDoc->SetTitle(title);
     }
     
+    // aSHEntry is now our currently-loaded document.
     mOSHE = aSHEntry;
 
+    // Clear the mLSHE reference to indicate document loading is done one
+    // way or another.
+    mLSHE = nsnull;
+
     // mEODForCurrentDocument is true here, so EndPageLoad will not fire
     // onload (we fire that below, in a special way so that the content window
     // does not see it).