ecore_wl2: Reset buffer_manager to NULL on failure

Calling this multiple times even after it fails the first time is a legit
thing now.  We'll be doing that when we want to test dmabuf at connection
start.

src/lib/ecore_wl2/ecore_wl2_buffer.c

@@ -433,6 +433,7 @@ err_bm:
    close(fd);
 err_drm:
    free(buffer_manager);
+   buffer_manager = NULL;
 err_alloc:
    return EINA_FALSE;
 }