builtin should be returned const

callers should never try to do any non-const operation with the
builtin public suffix list.

include/libpsl.h

@@ -64,7 +64,7 @@ psl_ctx_t *
 	psl_load_file(const char *fname);
 psl_ctx_t *
 	psl_load_fp(FILE *fp);
-psl_ctx_t *
+const psl_ctx_t *
 	psl_builtin(void);
 int
 	psl_is_public(const psl_ctx_t *psl, const char *domain);

src/psl.c

@@ -81,8 +81,8 @@ struct _psl_ctx_st {
 #include "suffixes.c"
 
 // references to this PSL will result in lookups to built-in data
-static psl_ctx_t
-	_builtin_psl;
+static const psl_ctx_t
+	_builtin_psl = { .suffixes = NULL, .suffix_exceptions = NULL, };
 
 static _psl_vector_t *_vector_alloc(int max, int (*cmp)(const _psl_entry_t *, const _psl_entry_t *))
 {
@@ -380,7 +380,7 @@ psl_ctx_t *psl_load_fp(FILE *fp)
 }
 
 // return built-in PSL structure
-psl_ctx_t *psl_builtin(void)
+const psl_ctx_t *psl_builtin(void)
 {
 	return &_builtin_psl;
 }

tests/test-is-public-builtin.c

@@ -70,7 +70,7 @@ static void test_psl(void)
 		{ "www.xn--czr694b", 1 },
 	};
 	unsigned it;
-	psl_ctx_t *psl;
+	const psl_ctx_t *psl;
 
 	if (psl_global_init() == 0) {
 		psl = psl_builtin();