bootstd: Init the size before reading extlinux file

The implementation in extlinux_pxe_getfile() does not pass a valid size
to bootmeth_read_file(), so this can fail if the uninited value happens to
be too small.

Fix this.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/boot/bootmeth_pxe.c b/boot/bootmeth_pxe.c
index ce986bd..8d489a1 100644
--- a/boot/bootmeth_pxe.c
+++ b/boot/bootmeth_pxe.c
@@ -31,6 +31,9 @@
 	int ret;
 
 	addr = simple_strtoul(file_addr, NULL, 16);
+
+	/* Allow up to 1GB */
+	*sizep = 1 << 30;
 	ret = bootmeth_read_file(info->dev, info->bflow, file_path, addr,
 				 sizep);
 	if (ret)