Fix memory leak in msc_rules_* C APIs

[weliu]

When msc_rules_* APIs fail, the error string will return to indicate the reason. It was a duplicate of an internal string. However, it will be duplicated again after those calls in Nginx connector. For example:

In ngx_http_modsecurity_merge_loc_conf():

rules = msc_rules_merge(c->rules_set, p->rules_set, &error);
if (rules < 0) {
return strdup(error);
}

Since the type of error is 'const char **', I believe that we should not duplicate it inside those APIs and let the caller decide how to handle it.

[p0pr0ck5]

Perhaps we should remove the duplicate strdup from the Nginx connector instead? The Apache connector does not call such a duplicate: https://github.com/SpiderLabs/ModSecurity-apache/blob/4e8854c46bd3c216d7834ab94605f45114794791/src/msc_config.c#L169-L183.

[weliu]

@p0pr0ck5 Since the Apache connector does not free the returned error string, there still exists memory leak if we only remove duplicate strdup from the Nginx connector.

[zimmerle]

Merged! Thanks!

[weliu]

@zimmerle Yes, I made a mistake, since the getParserError() return a temporary string, it is not safe for us to return its c_str. Sorry for the trouble I have made.

[zimmerle]

No problem. I dint notice at first sight as well.