the previous var wasn't being set so it went down the wrong code path when adjusting

the child list for creating the fieldset.
b=27151 r=kmcclusk

layout/base/nsCSSFrameConstructor.cpp

@@ -3714,6 +3714,7 @@ nsCSSFrameConstructor::ConstructFieldSetFrame(nsIPresShell*        aPresShell,
           break;
         }
       }
+      previous = child;
       child->GetNextSibling(&child);
     }
 

layout/html/style/src/nsCSSFrameConstructor.cpp

@@ -3714,6 +3714,7 @@ nsCSSFrameConstructor::ConstructFieldSetFrame(nsIPresShell*        aPresShell,
           break;
         }
       }
+      previous = child;
       child->GetNextSibling(&child);
     }