Fix/showsigpreview (#1042)

* ensure l:bufferlines is defined when using preview window

previously, when using the preview window to show signatures, the code
failed with an stacktrace like:

    "s:on_stdout client request on_notification() error",
    "Vim(return):E121: Undefined variable: l:bufferlines",
    "function <SNR>118_out_cb[2]..
    <SNR>107_on_stdout[79]..
    <SNR>92_request_on_notification[3]..
    <SNR>93_createNext[1]..
    <SNR>93_subscribeSourceCallback[2]..
    <lambda>738[1]..
    <SNR>94_handle_signature_help[48]..
    lsp#ui#vim#output#preview[39]..
    lsp#ui#vim#output#get_size_info, line 22"

This commit ensures that the required variable is properly defined in
all the situations (popup, nvim, preview window)

* removed superfluous return statement

Author: jerojasro

autoload/lsp/ui/vim/output.vim

@@ -284,8 +284,8 @@ function! lsp#ui#vim#output#get_size_info(winid) abort
     " Get size information while still having the buffer active
     let l:buffer = winbufnr(a:winid)
     let l:maxwidth = max(map(getbufline(l:buffer, 1, '$'), 'strdisplaywidth(v:val)'))
+    let l:bufferlines = 0
     if g:lsp_preview_max_width > 0
-      let l:bufferlines = 0
       let l:maxwidth = min([g:lsp_preview_max_width, l:maxwidth])
 
       " Determine, for each line, how many "virtual" lines it spans, and add
@@ -303,7 +303,6 @@ function! lsp#ui#vim#output#get_size_info(winid) abort
     endif
 
     return [l:bufferlines, l:maxwidth]
-  return [l:bufferlines, l:maxwidth]
 endfunction
 
 function! lsp#ui#vim#output#float_supported() abort