[PATCH] fix a segfault in sandbox.c if handle->dlcb is null

[Guillaume Benoit]

diff --git a/lib/libalpm/sandbox.c b/lib/libalpm/sandbox.c
index 1e96dc00..cdc01dc7 100644
--- a/lib/libalpm/sandbox.c
+++ b/lib/libalpm/sandbox.c
@@ -235,7 +235,9 @@ bool _alpm_sandbox_process_cb_download(alpm_handle_t 
*handle, int callback_pipe)
        ASSERT(read_from_pipe(callback_pipe, filename, filename_size) != -1, 
FREE(filename); return false);
        filename[filename_size] = '\0';

-       handle->dlcb(handle->dlcb_ctx, filename, type, &cb_data);
+       if(handle->dlcb) {
+               handle->dlcb(handle->dlcb_ctx, filename, type, &cb_data);
+       }
        FREE(filename);
        return true;
 }