include/openssl/e_os2.h: define last-resort SSIZE_MAX.

Reviewed-by: NRich Salz <rsalz@openssl.org>
Reviewed-by: NKurt Roeckx <kurt@roeckx.be>
(Merged from https://github.com/openssl/openssl/pull/6745)

include/openssl/e_os2.h

@@ -216,6 +216,8 @@ extern "C" {
 #   define OSSL_SSIZE_MAX SSIZE_MAX
 #  elif defined(_POSIX_SSIZE_MAX)
 #   define OSSL_SSIZE_MAX _POSIX_SSIZE_MAX
+#  else
+#   define OSSL_SSIZE_MAX ((ssize_t)(SIZE_MAX>>1))
 #  endif
 # endif