runtime: no need to protect key/value increments against end of bucket

After the key and value arrays, we have an overflow pointer.
So there's no way a past-the-end key or value pointer could point
past the end of the containing bucket.

So we don't need this additional protection.

Update #21459

Change-Id: I7726140033b06b187f7a7d566b3af8cdcaeab0b0
Reviewed-on: https://go-review.googlesource.com/56772
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Martin Möhrmann <moehrmann@google.com>
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: Avelino <t@avelino.xxx>

src/runtime/hashmap.go

@@ -1119,14 +1119,14 @@ func evacuate(t *maptype, h *hmap, oldbucket uintptr) {
 					typedmemmove(t.elem, dst.v, v)
 				}
 				dst.i++
-				// If we're at the end of the bucket, don't update k/v,
-				// to avoid pointers pointing past the end of the bucket.
-				if dst.i < bucketCnt {
+				// These updates might push these pointers past the end of the
+				// key or value arrays.  That's ok, as we have the overflow pointer
+				// at the end of the bucket to protect against pointing past the
+				// end of the bucket.
 				dst.k = add(dst.k, uintptr(t.keysize))
 				dst.v = add(dst.v, uintptr(t.valuesize))
 			}
 		}
-		}
 		// Unlink the overflow buckets & clear key/value to help GC.
 		if h.flags&oldIterator == 0 {
 			b = (*bmap)(add(h.oldbuckets, oldbucket*uintptr(t.bucketsize)))