encoding/gob: fewer decAlloc calls

Move decAlloc calls a bit higher in the call tree.
Cleans code marginally, improves speed marginally.
The benchmarks are noisy but the median time from
20 consective 1-second runs improves by about 2%.

LGTM=r
R=r
CC=golang-codereviews
https://golang.org/cl/105530043