Avoid NULL pointer dereference

Avoid NULL pointer dereference

Function readbuffer_gets() misses some of the initial checks of its
arguments. Not checking them can lead to a later NULL pointer
dereferences.

The checks are now unified with the checks in readbuffer_read()
function.

CLA: trivial
Fixes #23915

Signed-off-by: Radek Krejci <radek.krejci@oracle.com>
Reviewed-by: Tom Cosgrove <tom.cosgrove@arm.com>
Reviewed-by: Paul Dale <ppzgs1@gmail.com>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/23918)