Don't perform binary comparison of CRL issuer and CA subject

Previously, when checking whether a CRT was revoked through
one of the configured CRLs, the library would only consider
those CRLs whose `issuer` field binary-matches the `subject`
field of the CA that has issued the CRT in question. If those
fields were not binary equivalent, the corresponding CRL was
discarded.

This is not in line with RFC 5280, which demands that the
comparison should be format- and case-insensitive. For example:

- If the same string is once encoded as a `PrintableString` and
  another time as a `UTF8String`, they should compare equal.
- If two strings differ only in their choice of upper and lower case
  letters, they should compare equal.

This commit fixes this by using the dedicated x509_name_cmp()
function to compare the CRL issuer with the CA subject.

Fixes #1784.

library/x509_crt.c

@@ -1814,9 +1814,7 @@ static int x509_crt_verifycrl( mbedtls_x509_crt *crt, mbedtls_x509_crt *ca,
     while( crl_list != NULL )
     {
         if( crl_list->version == 0 ||
-            crl_list->issuer_raw.len != ca->subject_raw.len ||
+            x509_name_cmp( &crl_list->issuer, &ca->subject ) != 0 )
-            memcmp( crl_list->issuer_raw.p, ca->subject_raw.p,
-                    crl_list->issuer_raw.len ) != 0 )
         {
             crl_list = crl_list->next;
             continue;
@@ -1826,7 +1824,8 @@ static int x509_crt_verifycrl( mbedtls_x509_crt *crt, mbedtls_x509_crt *ca,
          * Check if the CA is configured to sign CRLs
          */
 #if defined(MBEDTLS_X509_CHECK_KEY_USAGE)
-        if( mbedtls_x509_crt_check_key_usage( ca, MBEDTLS_X509_KU_CRL_SIGN ) != 0 )
+        if( mbedtls_x509_crt_check_key_usage( ca,
+                                              MBEDTLS_X509_KU_CRL_SIGN ) != 0 )
         {
             flags |= MBEDTLS_X509_BADCRL_NOT_TRUSTED;
             break;