patch 8.2.4236: accessing freed memory

Problem:    Accessing freed memory.
Solution:   Set the bh_curr pointer to NULL.
diff --git a/src/getchar.c b/src/getchar.c
index 6602eea..cb8a354 100644
--- a/src/getchar.c
+++ b/src/getchar.c
@@ -110,6 +110,7 @@
 	vim_free(p);
     }
     buf->bh_first.b_next = NULL;
+    buf->bh_curr = NULL;
 }
 
 /*