sexpr_string: avoid leak on OOM error path

* src/xen/sexpr.c (sexpr_string): Free sexpr buffer upon allocation
failure.

src/xen/sexpr.c

@@ -119,6 +119,7 @@ sexpr_string(const char *str, ssize_t len)
     }
 
     if (ret->u.value == NULL) {
+        VIR_FREE(ret);
         return NULL;
     }