fix: check buffer is valid when getting name

## Details

Issue: #260

When logging info at the buffer level we get the file name for the
buffer to make the logs more useful. However if the buffer is invalid
this call to get file name will throw an exception.

Wrap the call to get file name in a check that buffer is valid. Return
some default string if it is not. Other parts of the ui.update flow will
validate the buffer and window and prevent any actual changes to invalid
buffers.

Author: MeanderingProgrammer

doc/render-markdown.txt

@@ -1,4 +1,4 @@
-*render-markdown.txt*         For 0.10.0         Last change: 2024 December 09
+*render-markdown.txt*         For 0.10.0         Last change: 2024 December 13
 
 ==============================================================================
 Table of Contents                          *render-markdown-table-of-contents*

lua/render-markdown/core/util.lua

@@ -128,8 +128,12 @@ end
 ---@param buf integer
 ---@return string
 function M.file_name(buf)
-    local file = vim.api.nvim_buf_get_name(buf)
-    return vim.fn.fnamemodify(file, ':t')
+    if vim.api.nvim_buf_is_valid(buf) then
+        local file = vim.api.nvim_buf_get_name(buf)
+        return vim.fn.fnamemodify(file, ':t')
+    else
+        return 'INVALID'
+    end
 end
 
 ---@param source string|integer

lua/render-markdown/health.lua

@@ -4,7 +4,7 @@ local state = require('render-markdown.state')
 local M = {}
 
 ---@private
-M.version = '7.7.3'
+M.version = '7.7.4'
 
 function M.check()
     M.start('version')