Fix implementation-defined integer conversion

"When an integer is demoted to a signed integer with smaller size, or an
unsigned integer is converted to its corresponding signed integer, if
the value cannot be represented the result is implementation-defined."
diff --git a/include/polarssl/x509_crt.h b/include/polarssl/x509_crt.h
index d534ad8..cb43262 100644
--- a/include/polarssl/x509_crt.h
+++ b/include/polarssl/x509_crt.h
@@ -256,7 +256,7 @@
  *                 POLARSSL_ERR_X509_CERT_VERIFY_FAILED if the chain was verified
  *                 but found to be invalid, in which case *flags will have one
  *                 or more BADCERT_XXX or POLARSSL_X509_BADCRL_XXX
- *                 flags set, or another error (and flags set to 0xffffffff)
+ *                 flags set, or another error (and flags set to -1)
  *                 in case of a fatal error encountered during the
  *                 verification process.
  */