http_ntlm: Fixed additional NSS initialisation call when decoding type-2

After commit 48d19acb7c the HTTP code would call Curl_nss_force_init()
twice when decoding a NTLM type-2 message, once directly and the other
through the call to Curl_sasl_decode_ntlm_type2_message().
ntlm: Moved NSS initialisation to base decode function