[Cgl] A bug in CglKnapsackCover

[Gleb Belov]

Dear developers,

I use Cbc to solve many small problems (column generation). There were
errors in CglKnapsackCover.cpp:
cover.truncate(cover.getNumElements()-1);
was called, whence the array was empty. I inserted checking but there
are other errors sometimes, they are not so regular. Saving the instance
and resolving separately works fine, obviously some randomization is 
used internally.

I had to abandon cover cuts because of this.