r.buffer: fix interger overflow (fixes https://trac.osgeo.org/grass/ticket/3620)

git-svn-id: https://svn.osgeo.org/grass/grass/trunk@73111 15284696-431f-4ddb-bdfa-cd5b030d7da7

raster/r.buffer/distance.h

@@ -50,7 +50,7 @@ extern double meters_to_grid;
 extern double ns_to_ew_squared;
 extern int count_rows_with_data;
 
-#define MAPINDEX(r,c) (r * window.cols + c)
+#define MAPINDEX(r,c) ((size_t)(r) * window.cols + (c))
 #define ZONE_INCR 2
 
 #define FEET_TO_METERS 0.3048