fix some crash on invalid layers

(cherry picked from commit 34488f8 & 8fbee12)

Author: jef-n

python/core/qgsvectorlayer.sip

@@ -284,7 +284,7 @@ class QgsVectorLayer : QgsMapLayer
     /** Removes a vector layer join */
     void removeJoin( const QString& joinLayerId );
 
-    const QList< QgsVectorJoinInfo >& vectorJoins() const;
+    const QList< QgsVectorJoinInfo > vectorJoins() const;
 
     /**
      * Add a new field which is calculated by the expression specified

src/core/qgsvectorfilewriter.cpp

@@ -1867,7 +1867,7 @@ QgsVectorFileWriter::WriterError QgsVectorFileWriter::writeAsVectorFormat( QgsVe
 
   QGis::WkbType wkbType = layer->wkbType();
 
-  if ( layer->providerType() == "ogr" )
+  if ( layer->providerType() == "ogr" && layer->dataProvider() )
   {
     QStringList theURIParts = layer->dataProvider()->dataSourceUri().split( "|" );
     QString srcFileName = theURIParts[0];

src/core/qgsvectorlayer.cpp

@@ -2840,7 +2840,7 @@ int QgsVectorLayer::fieldNameIndex( const QString& fieldName ) const
 
 bool QgsVectorLayer::addJoin( const QgsVectorJoinInfo& joinInfo )
 {
-  return mJoinBuffer->addJoin( joinInfo );
+  return mJoinBuffer && mJoinBuffer->addJoin( joinInfo );
 }
 
 void QgsVectorLayer::checkJoinLayerRemove( QString theLayerId )
@@ -2850,12 +2850,16 @@ void QgsVectorLayer::checkJoinLayerRemove( QString theLayerId )
 
 void QgsVectorLayer::removeJoin( const QString& joinLayerId )
 {
-  mJoinBuffer->removeJoin( joinLayerId );
+  if ( mJoinBuffer )
+    mJoinBuffer->removeJoin( joinLayerId );
 }
 
-const QList< QgsVectorJoinInfo >& QgsVectorLayer::vectorJoins() const
+const QList< QgsVectorJoinInfo > QgsVectorLayer::vectorJoins() const
 {
-  return mJoinBuffer->vectorJoins();
+  if ( mJoinBuffer )
+    return mJoinBuffer->vectorJoins();
+  else
+    return QList< QgsVectorJoinInfo >();
 }
 
 void QgsVectorLayer::addExpressionField( const QString& exp, const QgsField& fld )

src/core/qgsvectorlayer.h

@@ -647,7 +647,7 @@ class CORE_EXPORT QgsVectorLayer : public QgsMapLayer
     /** Removes a vector layer join */
     void removeJoin( const QString& joinLayerId );
 
-    const QList< QgsVectorJoinInfo >& vectorJoins() const;
+    const QList<QgsVectorJoinInfo> vectorJoins() const;
 
     /**
      * Add a new field which is calculated by the expression specified