Fix type error in PEM processing

The get_name() helper was using a variable of type size_t to hold the
result of BIO_gets(), but BIO_gets() returns int and makes use of negative
values to indicate error conditions.

Change the type of the local variable to match, and propagate that
through to other places in the file to avoid -Wsign-compare issues.

Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/5211)