Perl: fixed optimization in SSI command handler.

As the pointer to the first argument was tested instead of the argument
itself, array of arguments was always created, even if there were no
arguments.  Fix is to test args[0] instead of args.

Found by Coverity (CID 1356862).

src/http/modules/perl/ngx_http_perl_module.c

@@ -410,7 +410,7 @@ ngx_http_perl_ssi(ngx_http_request_t *r, ngx_http_ssi_ctx_t *ssi_ctx,
 
     args = &params[NGX_HTTP_PERL_SSI_ARG];
 
-    if (args) {
+    if (args[0]) {
 
         for (i = 0; args[i]; i++) { /* void */ }