fix for bug #801212 Assertion with ST_INTERSECTION on NULL values

        The ::val_str() method has to return NULL if it calculated
        the null_value, not just set the related flag.

per-file comments:
  mysql-test/r/gis-precise.result
fix for bug #801212 Assertion with ST_INTERSECTION on NULL values
        test result updated.
  mysql-test/t/gis-precise.test
fix for bug #801212 Assertion with ST_INTERSECTION on NULL values
        test case added.
  sql/item_geofunc.cc
fix for bug #801212 Assertion with ST_INTERSECTION on NULL values
        return NULL from the val_str if we get the null_value.

mysql-test/r/gis-precise.result

@@ -215,3 +215,6 @@ LINESTRING(0 0,1 1)
 SELECT Round(ST_Area(ST_BUFFER(MultipointFromText('MULTIPOINT(7 7,3 7,7 2,7 4 ,7 7)'), 3)), 5);
 Round(ST_Area(ST_BUFFER(MultipointFromText('MULTIPOINT(7 7,3 7,7 2,7 4 ,7 7)'), 3)), 5)
 78.68426
+SELECT ST_INTERSECTION(NULL, NULL);
+ST_INTERSECTION(NULL, NULL)
+NULL

mysql-test/t/gis-precise.test

@@ -115,5 +115,9 @@ SELECT astext(ST_UNION (
 #bug 801189 ST_BUFFER asserts if radius = 0
 SELECT astext(ST_BUFFER(LineStringFromText('LINESTRING(0 0,1 1)'),0));
 
-#buf 801199 Infinite recursion in Gcalc_function::count_internal with ST_BUFFER over MULTIPOINT
+#bug 801199 Infinite recursion in Gcalc_function::count_internal with ST_BUFFER over MULTIPOINT
 SELECT Round(ST_Area(ST_BUFFER(MultipointFromText('MULTIPOINT(7 7,3 7,7 2,7 4 ,7 7)'), 3)), 5);
+
+#bug 801212 Assertion with ST_INTERSECTION on NULL values
+SELECT ST_INTERSECTION(NULL, NULL);
+

sql/item_geofunc.cc

@@ -1014,7 +1014,10 @@ String *Item_func_spatial_operation::val_str(String *str_value)
 	!(g1= Geometry::construct(&buffer1, res1->ptr(), res1->length())) ||
 	!(g2= Geometry::construct(&buffer2, res2->ptr(), res2->length())) ||
 	g1->store_shapes(&trn) || g2->store_shapes(&trn))))
+  {
+    str_value= 0;
     goto exit;
+  }
 
   
   collector.prepare_operation();