The code was trying to interpret a uint32 n as a signed quantity
and then store it in an int. For this, int(n) currently works, but when
int becomes 64 bits one must write int(int32(n)) instead, to get
the 32-bit sign extension.

Update #2188.

R=golang-dev, nigeltao
CC=golang-dev
https://golang.org/cl/6551068