Support directories with "." in x509_load_serial()

Use `strrchr` to get a pointer to the last occurrence of `.` in the
path string, instead of the first one with `strchr`.  This prevent the
path to be wrongly split if it contains several `.`, and not only the
one for the extension.

Fixes https://github.com/openssl/openssl/issues/6489

.

CLA: trivial

Reviewed-by: Tim Hudson <tjh@openssl.org>
Reviewed-by: Matthias St. Pierre <Matthias.St.Pierre@ncp-e.com>
(Merged from https://github.com/openssl/openssl/pull/6566)