r.viewshed: fix small memory leak reported by coverity (merge from trunk, https://trac.osgeo.org/grass/changeset/62477)

git-svn-id: https://svn.osgeo.org/grass/grass/branches/releasebranch_7_0@62478 15284696-431f-4ddb-bdfa-cd5b030d7da7

raster/r.viewshed/grass.cpp

@@ -658,7 +658,8 @@ save_grid_to_GRASS(Grid * grid, char *filename, RASTER_MAP_TYPE type,
 	Rast_put_row(outfd, outrast, type);
     }				/* for i */
     G_percent(1, 1, 1);
-    
+
+    G_free(outrast);
     Rast_close(outfd);
     return;
 }