Fix PEM certificate loading that sometimes fails

Fix PEM certificate loading that sometimes fails

As described in https://github.com/openssl/openssl/issues/9187, the
loading of PEM certificates sometimes fails if a line of base64
content has the length of a multiple of 254.
The problem is in get_header_and_data(). When such a line with a
length of 254 (or a multiple) has been read, the next read will
only read a newline. Due to this get_header_and_data() expects to be
in the header not in the data area. This commit fixes that by checking
if lines have been read completely or only partially. In case of a
previous partial read, a newline will be ignored.

Reviewed-by: Dmitry Belyavskiy <beldmit@gmail.com>
Reviewed-by: Tomas Mraz <tmraz@fedoraproject.org>
Reviewed-by: Ben Kaduk <kaduk@mit.edu>
(Merged from https://github.com/openssl/openssl/pull/11741)