Check window size

Before setting showtabline option, make sure, there is enough room
fixes #863

autoload/airline/extensions/tabline/autoshow.vim

@@ -22,7 +22,9 @@ function! airline#extensions#tabline#autoshow#on()
   augroup airline_tabline_autoshow
     autocmd!
     if s:buf_min_count <= 0 && s:tab_min_count <= 1
-      set showtabline=2
+      if &lines > 3
+        set showtabline=2
+      endif
     else
       if s:show_buffers == 1
         autocmd BufEnter  * call <sid>show_tabline(s:buf_min_count, len(airline#extensions#tabline#buflist#list()))
@@ -40,7 +42,7 @@ endfunction
 
 function! s:show_tabline(min_count, total_count)
   if a:total_count >= a:min_count
-    if &showtabline != 2
+    if &showtabline != 2 && &lines > 3
       set showtabline=2
     endif
   else