runtime: fix 1-byte return during x.(T) for 0-byte T

runtime: fix 1-byte return during x.(T) for 0-byte T

The 1-byte write was silently clearing a byte on the stack.
If there was another function call with more arguments
in the same stack frame, no harm done.
Otherwise, if the variable at that location was already zero,
no harm done.
Otherwise, problems.

Fixes #8139.

LGTM=dsymonds
R=golang-codereviews, dsymonds
CC=golang-codereviews, iant, r
https://golang.org/cl/100940043