MDEV-4296 Assertion `n_linear_rings > 0' fails in Gis_polygon::centroid_xy.

Forgotten DBUG_ASSERT should be replaced with the 'return error'.

mysql-test/r/gis.result

@@ -1102,4 +1102,10 @@ NULL
 select geometryn(0x0100000000070000000100000001030000000200000000000000ffffff0f, 1);
 geometryn(0x0100000000070000000100000001030000000200000000000000ffffff0f, 1)
 NULL
+#
+# MDEV-4296 Assertion `n_linear_rings > 0' fails in Gis_polygon::centroid_xy
+#
+SELECT Centroid( AsBinary( LineString(Point(0,0), Point(0,0), Point(0,0) )));
+Centroid( AsBinary( LineString(Point(0,0), Point(0,0), Point(0,0) )))
+NULL
 End of 5.1 tests

mysql-test/t/gis.test

@@ -834,4 +834,9 @@ select envelope(0x0100000000030000000100000000000010);
 select geometryn(0x0100000000070000000100000001030000000200000000000000ffff0000, 1);
 select geometryn(0x0100000000070000000100000001030000000200000000000000ffffff0f, 1);
 
+--echo #
+--echo # MDEV-4296 Assertion `n_linear_rings > 0' fails in Gis_polygon::centroid_xy
+--echo #
+
+SELECT Centroid( AsBinary( LineString(Point(0,0), Point(0,0), Point(0,0) )));
 --echo End of 5.1 tests

sql/spatial.cc

@@ -972,13 +972,11 @@ int Gis_polygon::centroid_xy(double *x, double *y) const
   const char *data= m_data;
   bool first_loop= 1;
 
-  if (no_data(data, 4))
+  if (no_data(data, 4) ||
+      (n_linear_rings= uint4korr(data)) == 0)
     return 1;
-  n_linear_rings= uint4korr(data);
   data+= 4;
 
-  DBUG_ASSERT(n_linear_rings > 0);
-
   while (n_linear_rings--)
   {
     uint32 n_points, org_n_points;