Fix some code which causes warnings when compiled by /analyze tool

fpdfsdk/fpdf_flatten.cpp

 // Copyright 2014 PDFium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 // Original code copyright 2014 Foxit Software Inc. http://www.foxitsoftware.com
 
 #include "public/fpdf_flatten.h"
 
 #include <algorithm>
 
@@ skipping 138 matching lines @@
 
       break;
     }
     case BOTTOM: {
       for (int i = 0; i < nRects; i++)
         pArray[i] = CFX_FloatRect(array.GetAt(i)).bottom;
 
       break;
     }
     default:
-      break;
+      // Not reachable.
+      return 0.0f;
   }
   fRet = pArray[0];
   if (type == MAX) {
     for (int i = 1; i < nRects; i++)
       if (fRet <= pArray[i])
         fRet = pArray[i];
   } else {
     for (int i = 1; i < nRects; i++)
       if (fRet >= pArray[i])
         fRet = pArray[i];
@@ skipping 337 matching lines @@
     sStream += sTemp;
     pNewXObject->SetData(sStream.raw_str(), sStream.GetLength(), FALSE, FALSE);
   }
   pPageDict->RemoveAt("Annots");
 
   ObjectArray.RemoveAll();
   RectArray.RemoveAll();
 
   return FLATTEN_SUCCESS;
 }