patch 9.1.1474: missing out-of-memory check in mark.c

Problem:  missing out-of-memory check in mark.c
Solution: bail out, if mark_line() returns NULL
          (John Marriott)

closes: #17578

Signed-off-by: John Marriott <basilisk@internode.on.net>
Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/src/mark.c b/src/mark.c
index 9bab352..778110f 100644
--- a/src/mark.c
+++ b/src/mark.c
@@ -783,6 +783,11 @@
 	if (name == NULL && current)
 	{
 	    name = mark_line(p, 15);
+	    if (name == NULL)
+	    {
+		emsg(_(e_out_of_memory));
+		return;
+	    }
 	    mustfree = TRUE;
 	}
 	if (!message_filtered(name))