highlighter: do not skip empty c_sections

currently, if the matchstr() evaluates to '', it would skip the required
section, because bnr would be evaluated to zero and since there is no
buffer zero, skip that highlighting group.

Make sure, to only skip the highlighting group if the buffer number is
actually > 0

autoload/airline/highlighter.vim

@@ -256,7 +256,7 @@ function! airline#highlighter#highlight(modes, ...)
         " in the current tabpage
         if name[0:8] is# 'airline_c'
           let bnr = matchstr(name, 'airline_c\zs\d\+') + 0
-          if index(buffers_in_tabpage, bnr) == -1
+          if bnr > 0 && index(buffers_in_tabpage, bnr) == -1
             continue
           endif
         endif