Fix break for non-existing g:airline_theme

fixes #969

doc/airline.txt

@@ -82,8 +82,12 @@ values):
 <
 * themes are automatically selected based on the matching colorscheme. this
   can be overridden by defining a value. >
-  let g:airline_theme=
+  let g:airline_theme='dark'
 <
+  Note: Only the dark theme is distributed with vim-airline. For more themes,
+  checkout the vim-airline-themes repository
+  (github.com/vim-airline/vim-airline-themes)
+
 * if you want to patch the airline theme before it gets applied, you can
   supply the name of a function where you can modify the palette. >
   let g:airline_theme_patch_func = 'AirlineThemePatch'

plugin/airline.vim

@@ -7,8 +7,8 @@ endif
 let g:loaded_airline = 1
 
 let s:airline_initialized = 0
-let s:airline_theme_defined = 0
 function! s:init()
+  let s:airline_theme_defined = exists('g:airline_theme')
   if s:airline_initialized
     return
   endif
@@ -17,8 +17,11 @@ function! s:init()
   call airline#extensions#load()
   call airline#init#sections()
 
-  let s:airline_theme_defined = exists('g:airline_theme')
-  if s:airline_theme_defined || !airline#switch_matching_theme()
+  if !s:airline_theme_defined
+      let g:airline_theme = 'dark'
+      let s:airline_theme_defined = 1
+  endif
+  if !airline#switch_matching_theme()
     try
       let palette = g:airline#themes#{g:airline_theme}#palette
     catch