smbios: catch zero-length strings

smbios: catch zero-length strings

qemu may pass us zero-length strings for smbios fields, when starting
qemu this way  ...

	qemu -smbios type=1,version=,serial=test

... for example.

Today we don't specifically handle them and simply append them to the
string list.  Therefore we get two string-terminating zeros in a row.
Result is that we by accident create a end-of-entry marker in the middle
of the entry.

Fix this by handling zero-length strings like non-present strings.

Cc: armbru@redhat.com
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>