curl_global_sslset(): id == -1 is not necessarily an error

It is allowed to call that function with id set to -1, specifying the
backend by the name instead. We should imitate what is done further down
in that function to allow for that.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

lib/vtls/vtls.c

@@ -1313,7 +1313,9 @@ CURLsslset curl_global_sslset(curl_sslbackend id, const char *name,
     *avail = (const curl_ssl_backend **)&available_backends;
 
   if(Curl_ssl != &Curl_ssl_multi)
-    return id == Curl_ssl->info.id ? CURLSSLSET_OK : CURLSSLSET_TOO_LATE;
+    return id == Curl_ssl->info.id ||
+           (name && strcasecompare(name, Curl_ssl->info.name)) ?
+           CURLSSLSET_OK : CURLSSLSET_TOO_LATE;
 
   for(i = 0; available_backends[i]; i++) {
     if(available_backends[i]->info.id == id ||