Tidy and correct XDG var getenv() fallback

It's more correct for this function to get upset it's been passed
a variable name outside the XDG basedirs spec; a more general function,
in this case a backporting of getenv() from v8.1.1305.

1 files changed, 6 insertions, 11 deletions

diff --git a/vim/autoload/xdg.vim b/vim/autoload/xdg.vim
index 2b1e7c56..a0b88f82 100644
--- a/vim/autoload/xdg.vim
+++ b/vim/autoload/xdg.vim
@@ -11,18 +11,13 @@ let s:subdir = 'vim'
 
 function! s:Get(name) abort
   let name = a:name
-  if name !~# '^[A-Z][A-Z0-9_]*$'
-    throw 'Illegal env var name'
-  endif
-  let value = ''
-  execute 'let value = $'.name
-  if value !=# ''
-    return value
-  elseif has_key(s:defaults, name)
-    return s:defaults[name]
-  else
-    return ''
+  if !has_key(s:defaults, name)
+    throw 'Illegal XDG basedirs env var name'
   endif
+  let value = getenv#(name)
+  return value !=# v:null
+        \ ? value
+        \ : s:defaults[name]
 endfunction
 
 function! s:Absolute(path) abort