Fix: sessiond: bounded snapshot record fails when no streams exist

Fix: sessiond: bounded snapshot record fails when no streams exist

Attempting to record a snapshot with a `--max-size` fails when no
streams exist. For instance, attempting to record a snapshot for a
user space session when no applications are running will fail with the
following output:

Error: Invalid snapshot size. Cannot fit at least one packet per stream.
Error: Snapshot max size is invalid

The function get_session_nb_packets_per_stream() computes an
approximation of the number of packets to capture to honor the maximal
size specified. However, at the end, it doesn't distinguish between
'0' meaning that "no packets can be captured" (no streams exist) and
'0' meaning that "the max size is too small to accomodate one packet".

Those two cases can be distinguished by checking if the 'size_left' is
still the 'max_size', meaning that not even the size of one packet was
substracted from 'max_size'.

Reported-by: Simon Marchi <simark@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: I5caef9ce926bbc7143a90667749ffaed972590c1