Eliminate redundant NULL-checks in lower level sigchecking functions

Eliminate redundant NULL-checks in lower level sigchecking functions

- sigtd->data and dig checking (where needed) is done at
  rpmVerifySignature() level, dont bother double-checking
- Hash context is dup'ed, which CAN fail, so while we dont need
  to check the argument for non-null, the dup result needs to
  be checked for digests. For actual signatures the dup happens
  elsewhere, we dont need to check the argument for non-null here.