Document and enforce return values

In config_read_session_name(), moved the malloc to the beginning, to
avoid the fscanf() loop from trying to write to a NULL

In config_add_session_name(), dealt with another snprintf() occurrence

Signed-off-by: Daniel U. Thibault <daniel.thibault@drdc-rddc.gc.ca>
Signed-off-by: David Goulet <dgoulet@efficios.com>