Add code to check the validity of CollSeq objects during runtime. This code

Add code to check the validity of CollSeq objects during runtime.  This code
was not able to detect anomalies such as came up as a result of
ticket [e4a18565a36884b00edf66541f38c693827968ab] so it is put into a branch
for historical reference, with the intent of leaving it out of trunk.

FossilOrigin-Name: 68b23c3d41ce9303f42251d0c199181312e148f9