fix segfault or parsing error in '<!--# if expr="$var = /" -->'

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 79e89f1..f16c347 100644
--- a/src/http/modules/ngx_http_ssi_filter_module.c
+++ b/src/http/modules/ngx_http_ssi_filter_module.c
@@ -2376,7 +2376,7 @@
         p++;
     }
 
-    if (p < last && *p == '/') {
+    if (p < last - 1 && *p == '/') {
         if (*(last - 1) != '/') {
             goto invalid_expression;
         }