callgrind: fix setting call counter to zero if requested

callgrind: fix setting call counter to zero if requested

We did not set the call cost counters in CLG_(zero_bbcc) to zero,
which gets called by requests to set counters to zero
- via client request
- when entering a function if configured so
- via gdb monitor command (used by callgrind_control -z)

Counters are also set to zero after they are dumped out to a file.
This does not use CLG_(zero_bbcc), and was doing the right thing.

This fixes bug 392449.