Fix potential crash in vector gradient dialog info button

Note that this doesn't seem to be actually utilised by any
of the preinstalled gradients, but if users have created
a gradient with info then clicking the button would have
crashed QGIS.

Identified by clazy.

(cherry-picked from 84dade9)

Author: nyalldawson

src/gui/symbology-ng/qgsvectorgradientcolorrampv2dialog.cpp

@@ -101,8 +101,9 @@ void QgsVectorGradientColorRampV2Dialog::on_btnInformation_pressed()
   tableInfo->setRowCount( mRamp->info().count() );
   tableInfo->setColumnCount( 2 );
   int i = 0;
-  for ( QgsStringMap::const_iterator it = mRamp->info().constBegin();
-        it != mRamp->info().constEnd(); ++it )
+  QgsStringMap rampInfo = mRamp->info();
+  for ( QgsStringMap::const_iterator it = rampInfo.constBegin();
+        it != rampInfo.constEnd(); ++it )
   {
     if ( it.key().startsWith( "cpt-city" ) )
       continue;