Parser not terminating after `toml_err_set`, which may lead to assertion failure

[jidnjs]

Description

toml_err_set writes an error into the thread-local variable g_err and asserts that the previous error code was TOML_OK.

However, several code paths calls toml_err_set but does not stop parsing afterward.
When parsing continues and encounters another error, a second call to toml_err_set triggers the assertion, resulting in an abort.

static TOML_THREAD_LOCAL TomlErr g_err = {TOML_OK, (char*)"", TOML_TRUE};
...
TOML_INLINE void toml_err_set(TomlErrCode code, TOML_CONST char *format, ...)
{
    assert(g_err.code == TOML_OK);
    ...
    g_err.code = code;
    ...
}

Reproduction

1. create file named: repro_double_err_set with following contents:

[table]
unterminated_key = 
unterminated_key2 = 

2. run the parser

toml_load_filename("./repro_double_err_set");

Actual behavior

• The program aborts due to the assertion inside toml_err_set

example output:

src/toml.c:137: void toml_err_set(TomlErrCode, const char *, ...): Assertion `g_err.code == TOML_OK' failed.
Aborted (core dumped)

Expected behavior

• toml_load_filename should stop parsing after the first error and return NULL without crashing.

Fix suggestion

Certain codes need to return NULL or error code immediately after call to toml_err_set.

Add return NULL; at:

• line 802
• line 813

Use goto error; instead of goto cleanup;

• line 1585
• line 1590
• line 1595
• line 1620
• line 1631

Add goto error; at:

• line 1598

Cases needing multi-line updates

• line 1110

AS IS:

            toml_err_set(TOML_ERR_SYNTAX, "%s:%d:%d: invalid float");
            goto cleanup; // here

TO BE:

            toml_err_set(TOML_ERR_SYNTAX, "%s:%d:%d: invalid float");
            toml_string_free(str);
            return NULL;

• line 1638

AS IS

    toml_parse_key_value(self, real_table);

    goto cleanup;

TO BE

    if (toml_parse_key_value(self, real_table) == TOML_OK)
        goto cleanup;