fixes memleak : clear self referring VclPtr in MetricBox

Clears self referring VclPtr called mpField in MetricBox (inherited from FormatterBase class)
via dispose() method of MetricBox.

Change-Id: I375e03e814de3c6e47c36b1fc6f625b04521a5e2
Reviewed-on: https://gerrit.libreoffice.org/18674Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>

include/vcl/field.hxx

@@ -739,6 +739,8 @@ public:
     // Needed, because GetValue() with nPos hide these functions
     virtual sal_Int64       GetValue( FieldUnit eOutUnit ) const SAL_OVERRIDE;
     virtual sal_Int64       GetValue() const SAL_OVERRIDE;
+
+    virtual void            dispose() SAL_OVERRIDE;
 };
 
 

vcl/source/control/field.cxx

@@ -1776,6 +1776,12 @@ MetricBox::MetricBox( vcl::Window* pParent, WinBits nWinStyle ) :
     Reformat();
 }
 
+void MetricBox::dispose()
+{
+    MetricFormatter::SetField(NULL);
+    ComboBox::dispose();
+}
+
 Size MetricBox::CalcMinimumSize() const
 {
     Size aRet(calcMinimumSize(*this, *this));