b:airline_head sometimes undefined

Make sure, b:airline_head variable is set, also it is initially set in
this function earlier. But see #1171 for an error.

Therefore, before checking if the variable is empty, make sure it
exists.

fixes #1171

autoload/airline/extensions/branch.vim

@@ -141,6 +141,7 @@ function! airline#extensions#branch#head()
       endif
     endif
   else
+    let b:airline_head = get(b:, 'airline_head', '')
     for vcs in l:vcs_priority
       if has_key(l:heads, vcs)
         if !empty(b:airline_head)