Fix: report truncated files while reading

Fix: report truncated files while reading

When index files are available, this ensures we report the issue
properly before mapping a truncated packet rather than hitting a
SIGBUS.

For traces without index files, remove the check that was done at
index creation, and use the new check done when mapping the packet
instead. This ensures babeltrace can print stream data prior to the
truncated packet for this stream.

TODO: What is _not_ done in this patch is changing handling of truncated
packets: this should be treated as a warning rather than an error, so
processing of other streams continue.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
CC: Jonathan Rajotte-Julien <jonathan.rajotte-julien@efficios.com>
CC: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>