Fix CRL parsing to avoid infinite loop

This patch modifies the function mbedtls_x509_crl_parse() to ensure
that a CRL in PEM format with trailing characters after the footer does
not result in the execution of an infinite loop.

library/x509_crl.c

@@ -530,7 +530,7 @@ int mbedtls_x509_crl_parse( mbedtls_x509_crl *chain, const unsigned char *buf, s
 
             mbedtls_pem_free( &pem );
         }
-        else if( ret != MBEDTLS_ERR_PEM_NO_HEADER_FOOTER_PRESENT )
+        else if( is_pem )
         {
             mbedtls_pem_free( &pem );
             return( ret );