SSL: fixed ssl_buffer_size on SNI virtual hosts (ticket #1192).

SSL: fixed ssl_buffer_size on SNI virtual hosts (ticket #1192).

Previously, buffer size was not changed from the one saved during
initial ngx_ssl_create_connection(), even if the buffer itself was not
yet created.  Fix is to change c->ssl->buffer_size in the SNI callback.

Note that it should be also possible to update buffer size even in non-SNI
virtual hosts as long as the buffer is not yet allocated.  This looks
like an overcomplication though.