Mark logically dead code with corresponding comment.

Found by Coverity.

src/http/modules/ngx_http_ssi_filter_module.c

diff --git a/src/http/modules/ngx_http_ssi_filter_module.c b/src/http/modules/ngx_http_ssi_filter_module.c
index 13badac..eb286cc 100644
--- a/src/http/modules/ngx_http_ssi_filter_module.c
+++ b/src/http/modules/ngx_http_ssi_filter_module.c
@@ -1025,6 +1025,7 @@
         switch (state) {
 
         case ssi_start_state:
+            /* not reached */
             break;
 
         case ssi_tag_state: