cmd/gc: avoid pointer beyond array in range loop
cmd/gc: avoid pointer beyond array in range loop

This problem was discovered by reading the code.
I have not seen it in practice, nor do I have any ideas
on how to trigger it reliably in a test. But it's still worth
fixing.

TBR=ken2
CC=golang-codereviews
https://codereview.appspot.com/64370046

apparentlymart/golang,apparentlymart/golang,apparentlymart/golang,apparentlymart/golang,apparentlymart/golang,apparentlymart/golang,apparentlymart/golang