x509asn1: Fix SAN IP address verification

For IP addresses in the subject alternative name field, the length
of the IP address (and hence the number of bytes to perform a
memcmp on) is incorrectly calculated to be zero. The code previously
subtracted q from name.end. where in a successful case q = name.end
and therefore addrlen equalled 0. The change modifies the code to
subtract name.beg from name.end to calculate the length correctly.

The issue only affects libcurl with GSKit SSL, not other SSL backends.
The issue is not a security issue as IP verification would always fail.

Fixes #3102
Closes #3141