packet-cfm.c: Fix Bug 10385: "Ethernet OAM (CFM) frames including TLV's are wrongly decoded as malformed"

Regression introduced in ga06cccb (svn #50352) [Jul 3 16:43:40 2013]

Bug 10385
Change-Id: I3f5c5518a5804a1f31cac554323a6fa8d3d55d5d
Reviewed-on: https://code.wireshark.org/review/4078
Reviewed-by: Bill Meier <wmeier@newsguy.com>