Missing disposeAndClear in MsgBox basic function

Change-Id: Ida3d792a6f3797a6173b31b86a49b72ad7353340
Reviewed-on: https://gerrit.libreoffice.org/15717Reviewed-by: Michael Meeks <michael.meeks@collabora.com>
Tested-by: Michael Meeks <michael.meeks@collabora.com>

basic/source/runtime/methods.cxx

@@ -4649,6 +4649,7 @@ RTLFUNC(MsgBox)
         nMappedRet = nButtonMap[ nRet ];
     }
     rPar.Get(0)->PutInteger( nMappedRet );
+    pBox.disposeAndClear();
 }
 
 RTLFUNC(SetAttr)