fix(marks): adjust marks when unloading "nofile" buffer #39118

Problem:  Marks are not adjusted unloading a buffer that doesn't exist
          on disk. E.g. extmarks are still valid (and will be beyond the
          end of the buffer if the buffer is reloaded), even though the
          text is lost.
Solution: Adjust marks for a cleared buffer when unloading a buffer that
          doesn't exist on disk.

src/nvim/buffer.c

@@ -895,8 +895,16 @@ bool buf_freeall(buf_T *buf, int flags)
     unblock_autocmds();
   }
 
+  linenr_T count = buf->b_ml.ml_line_count;
   ml_close(buf, true);              // close and delete the memline/memfile
   buf->b_ml.ml_line_count = 0;      // no lines in buffer
+
+  // Ensure marks are adjusted for cleared buffer in case buffer not on disk:
+  // if it is reloaded the buffer will be empty.
+  if (bt_nofilename(buf) && !exiting) {
+    mark_adjust_buf(buf, 1, count, MAXLNUM, -count, false, kMarkAdjustNormal, kExtmarkNoUndo);
+  }
+
   if ((flags & BFA_KEEP_UNDO) == 0) {
     // free the memory allocated for undo
     // and reset all undo information