Check that default value exists, before accessing it

closes #1146

autoload/airline/extensions/default.vim

@@ -22,6 +22,9 @@ function! s:get_section(winnr, key, ...)
     endif
   endif
   let spc = g:airline_symbols.space
+  if !exists('g:airline_section_{a:key}')
+    return ''
+  endif
   let text = airline#util#getwinvar(a:winnr, 'airline_section_'.a:key, g:airline_section_{a:key})
   let [prefix, suffix] = [get(a:000, 0, '%('.spc), get(a:000, 1, spc.'%)')]
   return empty(text) ? '' : prefix.text.suffix