patch 8.2.4233: crash when recording and using Select mode

Problem:    Crash when recording and using Select mode.
Solution:   When deleting the last recorded character check there is something
            to delete.
diff --git a/src/getchar.c b/src/getchar.c
index c0dfc2b..49eb3d7 100644
--- a/src/getchar.c
+++ b/src/getchar.c
@@ -252,8 +252,11 @@
     static void
 delete_buff_tail(buffheader_T *buf, int slen)
 {
-    int len = (int)STRLEN(buf->bh_curr->b_str);
+    int len;
 
+    if (buf->bh_curr == NULL || buf->bh_curr->b_str == NULL)
+	return;  // nothing to delete
+    len = (int)STRLEN(buf->bh_curr->b_str);
     if (len >= slen)
     {
 	buf->bh_curr->b_str[len - slen] = NUL;