[MAJOR] call garbage collector when doing soft stop

When we're interrupted by another instance, it is very likely
that the other one will need some memory. Now we know how to
free what is not used, so let's do it.

Also only free non-null pointers. Previously, pool_destroy()
did implicitly check for this case which was incidentely
needed.
diff --git a/src/memory.c b/src/memory.c
index 589d1d2..8009227 100644
--- a/src/memory.c
+++ b/src/memory.c
@@ -103,6 +103,9 @@
 void pool_flush2(struct pool_head *pool)
 {
 	void *temp, *next;
+	if (!pool)
+		return;
+
 	next = pool->free_list;
 	while (next) {
 		temp = next;
@@ -141,11 +144,15 @@
 /*
  * This function destroys a pull by freeing it completely.
  * This should be called only under extreme circumstances.
+ * It always returns NULL, easing the clearing of the old pointer.
  */
-void pool_destroy2(struct pool_head *pool)
+void *pool_destroy2(struct pool_head *pool)
 {
-	pool_flush2(pool);
-	FREE(pool);
+	if (pool) {
+		pool_flush2(pool);
+		FREE(pool);
+	}
+	return NULL;
 }
 
 /* Dump statistics on pools usage.