r.drain: Add a rule to avoid an exception from r.path

git-svn-id: https://svn.osgeo.org/grass/grass/trunk@72863 15284696-431f-4ddb-bdfa-cd5b030d7da7

scripts/r.drain/r.drain.py

@@ -104,6 +104,9 @@
 #% description: The input raster map is a cost surface (direction surface must also be specified)
 #% guisection: Cost surface
 #%end
+#%rules
+#% required: start_coordinates, start_points
+#%end
 
 import os
 import sys