Simplify logging a bit

Simplify logging a bit
- precalculate the message length, relying on C99 vsnprintf() semantics
- generate the log message and rest of the record in rpmlog() already,
  and just pass the record to lower level to do actual logging
- also something was wrong in the old version as valgrind complained
  about illegal reads, that is also cured here
(cherry picked from commit 271069f90046994b005a469c80c2e150a41af9f3)