Fix getting global variable

Author: taqenoqo

syntax/markdown.vim

@@ -144,7 +144,7 @@ if get(b:, 'markdown_yaml_head', get(g:, 'markdown_yaml_head', main_syntax ==# '
   syn region markdownYamlHead start="\%^---$" end="^\%(---\|\.\.\.\)\s*$" keepend contains=@markdownYamlTop,@Spell
 endif
 
-if g:markdown_fenced_tex
+if get(g:, 'markdown_fenced_tex', 0)
   syn include syntax/tex.vim
   syn region markdownHighlighttex matchgroup=markdownCodeDelimiter start="\\\\(\ze[^ \n]" end="[^ \n]\zs\\\\)" keepend contains=@texMathZoneGroup
   syn region markdownHighlighttex matchgroup=markdownCodeDelimiter start="\\\\\[" end="\\\\\]" keepend contains=@texMathZoneGroup