Ignore "session_lifetime" if it can not be converted to a number

When "session_lifetime" can not be converted to a number the interval
becomes a NaN due to dividing it by 2. This NaN was "dragged" over all
the other mathematical operations and caused the heartbeat to be post
again and again due to an infinite loop with no pauses in "setInterval".
Now, the interval is set to the default value instead if the
"session_lifetime" can not be converted to a number.

Signed-off-by: Daniel Calviño Sánchez <danxuliu@gmail.com>