apps/enc.c: avoid signed integer overflow on bufsize assignment

apps/enc.c: avoid signed integer overflow on bufsize assignment

The calculated option value, while being long-typed, is not checked
for fitting into int-sized bufsize.  Avoid overflow by throwing error
if it is bigger than INT_MAX and document that behaviour.

Fixes: 7e1b7485706c "Big apps cleanup (option-parsing, etc)"
Resolves: https://scan5.scan.coverity.com/#/project-view/65248/10222?selectedIssue=1665149
References: https://github.com/openssl/project/issues/1362
Signed-off-by: Eugene Syromiatnikov <esyr@openssl.org>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
Reviewed-by: Paul Dale <ppzgs1@gmail.com>
Reviewed-by: Neil Horman <nhorman@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/28405)