pimd: reset packet size on tcp connection reset

pimd: reset packet size on tcp connection reset

If we were in the middle of a partial read when the tcp connection is
reset we were clearing the buffers but not the packet size. This can be
problematic when the connection is re-established.

There is no easy way to repro and test this without scale (and a timing
pattern that is hard to predict). So this change is mostly untested.

Ticket: CM-13852
Signed-off-by: Anuradha Karuppiah <anuradhak@cumulusnetworks.com>