internal/ucd: allow getting an out of bound field

UCD allows variable lenght rows.
Return an empty slice instead of setting an error when
a getField is out of bounds.

Change-Id: Ic6aa9043a32b38fae67037ac81442d88d812d5e1
Reviewed-on: https://go-review.googlesource.com/30390
Run-TryBot: Marcel van Lohuizen <mpvl@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Nigel Tao <nigeltao@golang.org>