minor type-safety and variable init changes in init.cpp

- explicit init of pcoinsdbview and pwalletMain (even if not needed, as
  globals are init to NULL, it seems cleaner)
- remove check if (pwalletMain) in Shutdown() as delete is valid even if
  pwalletMain is NULL
- use correct type (%d / %u) in log and help messages
This is unnecessary, tinyformat doesn't look at 'd'/'u' but looks at the type of the parameter.