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/expl.c

@@ -119,7 +119,7 @@ explore(void)
 	pr("Only exploring with %d.\n", amount);
     }
 
-    if (!want_to_abandon(&sect, vtype, amount, 0)) {
+    if (!want_to_abandon(&sect, vtype, amount, NULL)) {
 	pr("Explore cancelled.\n");
 	return RET_FAIL;
     }