media: drivers/media/v4l2-core/v4l2-h264 : add detection of null pointers

When the pointer variable is judged to be null, null is returned
directly.

[hverkuil: fix two checkpatch warnings]
Signed-off-by: Dong Chuanjian <chuanjian@nfschina.com>
Acked-by: Nicolas Dufresne <nicolas.dufresne@collabora.com>
Fixes: d3f756ad ("media: v4l2: Trace calculated p/b0/b1 initial reflist")
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>

drivers/media/v4l2-core/v4l2-h264.c

@@ -305,6 +305,8 @@ static const char *format_ref_list_p(const struct v4l2_h264_reflist_builder *bui
 	int n = 0, i;
 
 	*out_str = kmalloc(tmp_str_size, GFP_KERNEL);
+	if (!(*out_str))
+		return NULL;
 
 	n += snprintf(*out_str + n, tmp_str_size - n, "|");
 
@@ -343,6 +345,8 @@ static const char *format_ref_list_b(const struct v4l2_h264_reflist_builder *bui
 	int n = 0, i;
 
 	*out_str = kmalloc(tmp_str_size, GFP_KERNEL);
+	if (!(*out_str))
+		return NULL;
 
 	n += snprintf(*out_str + n, tmp_str_size - n, "|");