runtime: fix map memory leak

runtime: fix map memory leak

The map implementation was using the C idiom of using
a pointer just past the end of its table as a limit pointer.
Unfortunately, the garbage collector sees that pointer as
pointing at the block adjacent to the map table, pinning
in memory a block that would otherwise be freed.

Fix by making limit pointer point at last valid entry, not
just past it.

Reviewed by Mike Burrows.

R=golang-dev, bradfitz, lvd, r
CC=golang-dev
https://golang.org/cl/5158045