e1000: Initialize helper variables in em_newitr()

Due to races with the threaded transmit and receive paths, it's possible
to have r/tx_bytes != 0 && r/tx_packets == 0, in which case the maximum
byte count could be left uninitialized. Initialize them to zero to
handle this case.

PR: 286819
Reviewed by: kbowling
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D50416

(cherry picked from commit e0267657f3965a56d877075fe3d4d41b8afb2faf)