lib/ukrandom: Validate getrandom input & rm assert

lib/ukrandom: Validate getrandom input & rm assert

This change removes the assert on the buffer argument to the getrandom
syscall and replaces it with defined input validation, based on observed
behavior in Linux:
- if the requested number of bytes is 0, getrandom shortcuts to success
- if buffer is NULL and > 0 bytes requested, return -EFAULT instead of
  crashing

Signed-off-by: Andrei Tatar <andrei@unikraft.io>
Approved-by: Michalis Pappas <michalis@unikraft.io>
Reviewed-by: Michalis Pappas <michalis@unikraft.io>
GitHub-Closes: #1637