Fix --strict-warnings build

Fix --strict-warnings build

The %zd format corresponds to ssize_t which is used for
function to either return a valid size or a negative value
to indicate an error. Since size_t is in [-1,SSIZE_MAX] it
is not a portable way to represent a pointer diff. For
the %td format which corresponds to ptrdiff_t is C11,
we chose to cast to long instead as it is already done
in other places.

Signed-off-by: Patrick Steuer <patrick.steuer@de.ibm.com>
Reviewed-by: Richard Levitte <levitte@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/10335)