Fix some invalid use of sscanf

Fix some invalid use of sscanf

sscanf can return -1 on an empty input string. We need to appropriately
handle such an invalid case.

The instance in OSSL_HTTP_parse_url could cause an uninitialised read of
sizeof(unsigned int) bytes (typically 4). In many cases this uninit read
will immediately fail on the following check (i.e. if the read value
>65535).

If the top 2 bytes of a 4 byte unsigned int are zero then the value will
be <=65535 and the uninitialised value will be returned to the caller and
could represent arbitrary data on the application stack.

The OpenSSL security team has assessed this issue and consider it to be
a bug only (i.e. not a CVE).

Reviewed-by: Todd Short <todd.short@me.com>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
Reviewed-by: Shane Lontis <shane.lontis@oracle.com>
(Merged from https://github.com/openssl/openssl/pull/22961)