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

Otherwise an invalid specifier iterates over uninitialized data.

Fixes a bug introduced by 0b40688d18 (v254).

(cherry picked from commit 32b8065e876c6f89f55b1bb30eeb442d3921fb3a)
(cherry picked from commit 5dc97968d6ef732508311ecff555b305a05b6fe5)

src/core/load-fragment.c

@@ -2789,6 +2789,7 @@ int config_parse_environ(
                 COMMON_CREDS_SPECIFIERS(ltype),
                 { 'h', specifier_user_home,  NULL },
                 { 's', specifier_user_shell, NULL },
+                {}
         };
 
         for (const char *p = rvalue;; ) {