bat: really skip analysis of the first period and update related comment

bat: really skip analysis of the first period and update related comment

Prior to this change bat/analyze.c would skip the last period of the recording, contrary to
what the comment in the code which stated the first period was meant to be skipped.

The comment has been updated to state that both the first and last period are skipped and the code
has been updated to match.

Closes: https://github.com/alsa-project/alsa-utils/pull/237
Signed-off-by: Delio Brignoli <dbrignoli@audioscience.com>
Signed-off-by: Jaroslav Kysela <perex@perex.cz>