x509v3/v3_purp.c: resolve Thread Sanitizer nit.

Reviewed-by: NPaul Dale <paul.dale@oracle.com>
Reviewed-by: NRich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/6786)

crypto/x509v3/v3_purp.c

@@ -351,12 +351,8 @@ static void x509v3_cache_extensions(X509 *x)
     ASN1_BIT_STRING *ns;
     EXTENDED_KEY_USAGE *extusage;
     X509_EXTENSION *ex;
-
     int i;
 
-    if (x->ex_flags & EXFLAG_SET)
-        return;
-
     CRYPTO_THREAD_write_lock(x->lock);
     if (x->ex_flags & EXFLAG_SET) {
         CRYPTO_THREAD_unlock(x->lock);