start.sh SAML Extension load logic incomplete

The logic contained within the current start.sh script is incomplete when deciding to load the guacamole-auth-saml extension.

As per the documentation the SAML Extension can be configured by setting either `$SAML_IDP_METADATA_URL` or `$SAML_ENTITY_ID` and `$SAML_CALLBACK_URL`

At line 1049 in script start.sh the SAML Extension is only loaded if `$SAML_IDP_METADATA_URL` is present.

if [ -n "$SAML_IDP_METADATA_URL" ]; then
    associate_saml
    INSTALLED_AUTH="$INSTALLED_AUTH saml"
fi

This should include logic similar to line 754 in the same start.sh

if [ -z "$SAML_IDP_METADATA_URL" ] && [ -z "$SAML_ENTITY_ID" -o -z "$SAML_CALLBACK_URL" ]