3.0.0 compile failure when SASL is enabled but not TLS

[Joshua Root]

In the 3.0.0 release, line 4028 of src/smtpd/smtpd.c references
state->tls_context, but unlike other such references in this file, it is
not wrapped in an #ifdef USE_TLS. This causes compilation to fail when
building with SASL support but without TLS:

smtpd.c:4028:13: error: no member named 'tls_context' in 'SMTPD_STATE'
        if (state->tls_context == 0)            /* TLS from XCLIENT
proxy? */
            ~~~~~  ^

This is a regression since 2.11.3. I won't attempt a patch since I'm not
familiar with the code.

- Josh