fix: handle page fault errors when reading envs

[RonFed]

This PR slightly changes the algorithm for env prefix filtering:

• If we reached a NULL pointer in the envp vector we return and not pass a process exec event. This happens often when we successfully scan the env vector of an un-relevant process.
• If we read an env pointer successfully but fail to de-reference it and read the underlying string - we have no certainty about the envs - and this can happen if the memory referenced by the env pointer is not paged in yet. In this case we continue as if we found a relevant env prefix.

emergency-ticket id: EMR-1