ftp: Don't assert on a valid connection handle in Read()

When seeking, if the reconnection fails, the connection handle will be
NULL. Instead, try to reconnect and signal EOF if this can't be done.

(cherry picked from commit dde0b69477)
Signed-off-by: Hugo Beauzée-Luyssen <hugo@beauzee.fr>

modules/access/ftp.c

@@ -900,7 +900,8 @@ static ssize_t Read( stream_t *p_access, void *p_buffer, size_t i_len )
 {
     access_sys_t *p_sys = p_access->p_sys;
 
-    assert( p_sys->data != NULL );
+    if( p_sys->data == NULL )
+        return 0;
     assert( !p_sys->out );
 
     ssize_t i_read = vlc_tls_Read( p_sys->data, p_buffer, i_len, false );