mrcpv2: don't use an uninitialized buffer as a string

I have no idea what the original intent was with this magical
otherwise-totally-unused buffer, but it clearly serves no purpose now. Stop
reading garbage from it, and just read the data in the packet which is what the
field name suggests.

Bug: 10510
Change-Id: I05d0b98c04e59ea70247811168c4c8a64861f43d
Reviewed-on: https://code.wireshark.org/review/4354
Reviewed-by: Michael Mann <mmann78@netscape.net>