Fix coverity-1604661

Fix coverity-1604661

Coverity called out an error in asn1parse_main, indicating that the
for(;;) loop which repeatedly reads from a bio and updates the length
value num, may overflow said value prior to exiting the loop.

We could probably call this a false positive, but on very large PEM
file, I suppose it could happen, so just add a check to ensure that num
doesn't go from a large positive to a large negative value inside the
loop

Fixes openssl/private#571

Reviewed-by: Tom Cosgrove <tom.cosgrove@arm.com>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/24910)