Don't use 0 as null pointer constant, part 1

Use NULL instead of 0, for clarity.  Except in pointer comparisons;
leave that to the next two commits.

src/lib/commands/stre.c

@@ -82,7 +82,7 @@ stre(void)
 	    pr("%7s", "");
 
 	if (def->sct_type != SCT_MOUNT)
-	    r_total = att_reacting_units(def, 0, 0, &dummy, 9999999);
+	    r_total = att_reacting_units(def, NULL, 0, &dummy, 9999999);
 	else
 	    r_total = 0.0;
 	def->own = 0;