Fix: use of uninitialized 'nb_pipes_fd'

nb_pipes_fd can be used uninitialized in the consumers' data polling
threads if 'consumer_data.need_update' is false.

In practice, this is not a problem as 'consumer_data.need_update'
is true on the first execution of the thread's loop.

Reported-by: Coverity Scan (1387046 Uninitialized scalar variable)
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>