Better handling of get_activity_position() return value

Better handling of get_activity_position() return value

Function get_activity_position() can return -1 on error, ie. when
corresponding activity hasn't been found in array.
This return code was sometimes not tested because we knew that the
activity existed. So we directly used the return code as array index.
Strengthen things now: If such an activity is not found then display an
internal error message and exit.

Signed-off-by: Sebastien GODARD <sysstat@users.noreply.github.com>