Fix use of Python assert() with respect to optimization.

Fix use of Python assert() with respect to optimization.

Fix a number of places where bup's assertions had material
side-effects, or where other code expected to see the AssertionError,
neither of which happen when optimization is enabled.

Reported-by: Jon Dowland <jmtd@debian.org>
Signed-off-by: Rob Browning <rlb@defaultvalue.org>