BUG/MEDIUM: server: do not use default SNI if manually set

BUG/MEDIUM: server: do not use default SNI if manually set

A new server feature "sni-auto" has been introduced recently. The
objective is to automatically set the SNI value to the host header if no
SNI is explicitely set.

  668916c1a2fc2180028ae051aa805bb71c7b690b
  MEDIUM: server/ssl: Base the SNI value to the HTTP host header by default

There is an issue with it : server SNI is currently always overwritten,
even if explicitely set in the configuration file. Adjust
check_config_validity() to ensure the default value is only used if
<sni_expr> is NULL.

This issue was detected as a memory leak on <sni_expr> was reported when
SNI is explicitely set on a server line.

This patch is related to github feature request #3081.

No need to backport, unless the above patch is.