PEM_def_callback(): don't loop because of too short password given

That error is already caught by EVP_read_pw_string_min, and causes
this function to return -1, so the code detecting too short passwords
in this function is practically dead.

Fixes #5465

Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/6080)