[processing] repair non-working random extract algorithms (fix #11571)

Author: alexbruy

python/plugins/processing/algs/qgis/RandomExtract.py

@@ -85,9 +85,8 @@ def processAlgorithm(self, progress):
 
         selran = random.sample(xrange(0, featureCount), value)
 
-        output = self.getOutputFromName(self.OUTPUT)
-        writer = output.getVectorWriter(layer.fields(),
-                layer.geometryType(), layer.crs())
+        writer = self.getOutputFromName(self.OUTPUT).getVectorWriter(
+            layer.pendingFields().toList(), layer.wkbType(), layer.crs())
 
         for (i, feat) in enumerate(features):
             if i in selran:

python/plugins/processing/algs/qgis/RandomExtractWithinSubsets.py

@@ -91,10 +91,8 @@ def processAlgorithm(self, progress):
                         value and try again.")
             value = value / 100.0
 
-
-        output = self.getOutputFromName(self.OUTPUT)
-        writer = output.getVectorWriter(layer.fields(),
-                layer.geometryType(), layer.crs())
+        writer = self.getOutputFromName(self.OUTPUT).getVectorWriter(
+            layer.pendingFields().toList(), layer.wkbType(), layer.crs())
 
         selran = []
         current = 0