The code filling the cache assumes that s points to the end of the
key grip. We have to set it back after we have parsed the serial
number (where we temporarily set s to the start of the serial number).

GnuPG-bug-id: 7914

Without the fix no serial numbers are listed because the hexgrips in the keyinfo_cache all end with " T " which breaks the lookup of the keygrips in the cache.

Run gpgsm -k --with-secret --with-colons. The lines starting with crt should contain the serial numbers of the smartcards that the certificates are stored on in field 15 of the colon-separated list.