vtls: fix default SSL backend as a fallback

vtls: fix default SSL backend as a fallback

- Use build-time CURL_DEFAULT_SSL_BACKEND as a fallback when environment
  variable CURL_SSL_BACKEND contains a backend that is unavailable.

Prior to this change if CURL_SSL_BACKEND was set then
CURL_DEFAULT_SSL_BACKEND was ignored even if the backend of the former
was unavailable. In that case libcurl would instead select the first
available backend in the list of backends.

Bug: https://github.com/curl/curl/issues/16076#issuecomment-2617354254
Reported-by: Jeroen Ooms
Closes https://github.com/curl/curl/pull/16108