selfree maintains a cache of Select structures for several sizes. In
newselect, we'll use an entry from the cache if one is found. However,
the Scase structures corresponding to a send may have been allocated
for the wrong size. In this case we'll write off the end of the Scase
into random memory and, generally, read some amount of junk in the
receive.

This patch fixes the issue by removing the cache, on the advice of
rsc.

R=rsc
CC=go-dev
http://go/go-review/1016002