sandbox: p2sb: Silence compiler warning

Some compilers produce a warning about 'child' being used before init.
Silence this by setting to NULL at the start.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>

drivers/misc/p2sb_emul.c

@@ -215,7 +215,7 @@ static int sandbox_p2sb_emul_map_physmem(struct udevice *dev,
 					 void **ptrp)
 {
 	struct p2sb_emul_priv *priv = dev_get_priv(dev);
-	struct udevice *child;
+	struct udevice *child = NULL;  /* Silence compiler warning */
 	unsigned int offset;
 	int barnum;
 	int ret;