load-fragment: terminate the specifier table (#34421)

Otherwise an invalid specifier iterates over uninitialized data.

Fixes a bug introduced by 0b40688d18 (v254).

src/core/load-fragment.c

@@ -2728,6 +2728,7 @@ int config_parse_environ(
                 COMMON_CREDS_SPECIFIERS(ltype),
                 { 'h', specifier_user_home,  NULL },
                 { 's', specifier_user_shell, NULL },
+                {}
         };
 
         for (const char *p = rvalue;; ) {