Refine the timing of when the ipbind table is initialized.

Turns out there are modules such as `mod_autohost` which will create ipbind
entries _before_ the rest of the core engine adds its entries, such as in
a `core.postparse` event listener.

Which was fine -- until [this commit](proftpd@9d55ce4) unconditionally cleared that ipbind
table, effectively erasing the efforts of such modules.  Oops.

The intent is to ensure that, when searching for relevant ipbind entries, the
table has been properly cleared of any junk values.  Definitely a desirable
starting state.  We just have to be more careful about how we do it.

src/bindings.c

@@ -31,12 +31,25 @@ extern xaset_t *server_list;
 extern server_rec *main_server;
 
 static pr_ipbind_t *ipbind_table[PR_BINDINGS_TABLE_SIZE];
+static int ipbind_table_initialized = FALSE;
+
 static pool *binding_pool = NULL;
 static pr_ipbind_t *ipbind_default_server = NULL,
                    *ipbind_localhost_server = NULL;
 
 static const char *trace_channel = "binding";
 
+static void trace_ipbind_table(void);
+
+static void init_ipbind_table(void) {
+  if (ipbind_table_initialized == TRUE) {
+    return;
+  }
+
+  memset(ipbind_table, 0, sizeof(ipbind_table));
+  ipbind_table_initialized = TRUE;
+}
+
 /* Server cleanup callback function */
 static void server_cleanup_cb(void *conn) {
   *((conn_t **) conn) = NULL;
@@ -451,6 +464,9 @@ int pr_ipbind_create(server_rec *server, const pr_netaddr_t *addr,
     return -1;
   }
 
+  /* Ensure the ipbind table has been initialized. */
+  init_ipbind_table();
+
   i = ipbind_hash_addr(addr);
   pr_trace_msg(trace_channel, 29, "hashed address '%s' to index %u",
     pr_netaddr_get_ipstr(addr), i);
@@ -566,6 +582,9 @@ pr_ipbind_t *pr_ipbind_find(const pr_netaddr_t *addr, unsigned int port,
     return NULL;
   }
 
+  /* Ensure the ipbind table has been initialized. */
+  init_ipbind_table();
+
   i = ipbind_hash_addr(addr);
 
   for (ipbind = ipbind_table[i]; ipbind; ipbind = ipbind->ib_next) {
@@ -1186,6 +1205,7 @@ void free_bindings(void) {
   }
 
   memset(ipbind_table, 0, sizeof(ipbind_table));
+  ipbind_table_initialized = FALSE;
 
   /* Mark all listening conns as "unclaimed"; any that remaining unclaimed
    * after init_bindings() can be closed.
@@ -1460,7 +1480,8 @@ static int init_standalone_bindings(void) {
   server_rec *serv = NULL;
   unsigned char *default_server = NULL, is_default = FALSE;
 
-  memset(ipbind_table, 0, sizeof(ipbind_table));
+  /* Ensure the ipbind table has been initialized. */
+  init_ipbind_table();
 
   /* If a port is set to zero, the address/port is not bound to a socket
    * at all.