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/move.c

@@ -140,7 +140,7 @@ move(void)
 	}
     }
 
-    if (!istest && !want_to_abandon(&sect, vtype, amount, 0)) {
+    if (!istest && !want_to_abandon(&sect, vtype, amount, NULL)) {
 	pr("Move cancelled.\n");
 	return RET_FAIL;
     }