SPDY: Fix the display of COL_INFO...

Clear the COL_INFO before start to dissect SPDY PDU

Change-Id: I7a22515f1c2c89b5647003320ff455a74fbce42a
Ping-Bug: 10566
Reviewed-on: https://code.wireshark.org/review/4636
Reviewed-by: Michael Mann <mmann78@netscape.net>
(cherry picked from commit 2c23c7e79d3cf3fc82af67d706981dc1e7abb328)
Reviewed-on: https://code.wireshark.org/review/4647