Accessing a NULL pointer in ex_cmds.c

[Yegappan Lakshmanan]

Hi,

Running Vim with the clang undefined behavior sanitizer (UBSan) finds
an access using a NULL pointer in ex_cmds.c. The attached patch fixes
this issue.

- Yegappan

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diff --git a/src/ex_cmds.c b/src/ex_cmds.c
index 206ead1..2b43b7c 100644
--- a/src/ex_cmds.c
+++ b/src/ex_cmds.c
@@ -3967,7 +3967,8 @@ do_ecmd(
                     * <VN> We could instead free the synblock
                     * and re-attach to buffer, perhaps.
                     */
-                   if (curwin->w_s == &(curwin->w_buffer->b_s))
+                   if (curwin->w_buffer != NULL &&
+                           curwin->w_s == &(curwin->w_buffer->b_s))
                        curwin->w_s = &(buf->b_s);
 #endif
                    curwin->w_buffer = buf;