simplification of radicals



A few versions ago, I think the following simplified to zero; now
it doesn't:

 (%i6) sqrt(3) * sqrt(17) - sqrt(51);
 (%o6) sqrt(3) sqrt(17) - sqrt(51)

Both CVS Maxima and 5.15.0 will not simplify %i6 to zero. If this is a new
bug, it's sad that the test suite didn't catch it.

Barton