Use tvb_new_subset_length() rather than tvb_new_subset().

We rarely, if ever, want to, or need to, explicitly set the captured
length of a packet; we want to set the *reported* length and let the
tvbuff code figure out how much of that data was actually captured.

Change-Id: I9b93d296197989f677d888b10954589dfc1edc59
Reviewed-on: https://code.wireshark.org/review/4138
Reviewed-by: Guy Harris <guy@alum.mit.edu>