srzip: Fix handling of analog channels with index 0

srzip: Fix handling of analog channels with index 0

0 is a valid index for a channel. Using it as the value for the terminating
entry of analog_index_map causes zip_append_analog() to falsely assume that
no channel was found when a packet for a channel with index 0 was received.
This prevents the data for the channel to be added to the sigrok session
file.

Instead use -1, which is not a valid channel index, as
the terminating entry value.

Signed-off-by: Lars-Peter Clausen <redacted>