*.[ch] refactoring: make use of the FREE_AND_NULL() macro

Replace occurrences of `free(ptr); ptr = NULL` which weren't caught by
the coccinelle rule. These fall into two categories:

 - free/NULL assignments one after the other which coccinelle all put
   on one line, which is functionally equivalent code, but very ugly.

 - manually spotted occurrences where the NULL assignment isn't right
   after the free() call.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

prio-queue.c

@@ -27,10 +27,9 @@ void prio_queue_reverse(struct prio_queue *queue)
 
 void clear_prio_queue(struct prio_queue *queue)
 {
-	free(queue->array);
+	FREE_AND_NULL(queue->array);
 	queue->nr = 0;
 	queue->alloc = 0;
-	queue->array = NULL;
 	queue->insertion_ctr = 0;
 }