fix 4 separate new bugs in BitmapMosaicFilter

coverity#1435274 Dereference after null check
coverity#1435276 Resource leak
coverity#1435278 Resource leak
and on success it returned its input, throwing
away the result of its calculation

commit 63a71678
Author: Chris Sherlock <chris.sherlock79@gmail.com>
Date:   Fri Apr 20 20:39:48 2018 +1000

    vcl: ImplMosaic() -> BitmapMosaicFilter

    Change-Id: Ia0910ae9166c4eb6b870ab25db761bc1703fec68
    Reviewed-on: https://gerrit.libreoffice.org/53203Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Tomaž Vajngerl <quikee@gmail.com>

Change-Id: I4eb74ef279b2ead268d07ac572ab76cd911dacb6
Reviewed-on: https://gerrit.libreoffice.org/53807Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>

vcl/source/bitmap/BitmapMosaicFilter.cxx

@@ -166,20 +166,27 @@ BitmapEx BitmapMosaicFilter::execute(BitmapEx const& rBitmapEx)
 
         Bitmap::ReleaseAccess(pReadAcc);
 
-        if (bRet)
+        if (pNewBmp)
         {
-            const MapMode aMap(aBitmap.GetPrefMapMode());
-            const Size aPrefSize(aBitmap.GetPrefSize());
+            Bitmap::ReleaseAccess(pWriteAcc);
 
-            aBitmap = *pNewBmp;
+            if (bRet)
+            {
+                const MapMode aMap(aBitmap.GetPrefMapMode());
+                const Size aPrefSize(aBitmap.GetPrefSize());
+
+                aBitmap = *pNewBmp;
+
+                aBitmap.SetPrefMapMode(aMap);
+                aBitmap.SetPrefSize(aPrefSize);
+            }
 
-            aBitmap.SetPrefMapMode(aMap);
-            aBitmap.SetPrefSize(aPrefSize);
+            delete pNewBmp;
         }
     }
 
     if (bRet)
-        return rBitmapEx;
+        return BitmapEx(aBitmap);
 
     return BitmapEx();
 }