Fix ineffective recycling (#18952)

Recycle would always be called on the dummy value `any(newRT())` instead of the actual value given to the recycle function.

Caught by race tests, but mostly harmless, except for reduced perf.

Other minor cleanups. Introduced in #18940 (unreleased)

internal/grid/types.go

@@ -235,7 +235,6 @@ func (b *Bytes) UnmarshalMsg(bytes []byte) ([]byte, error) {
 		copy(*b, val)
 	} else {
 		if cap(*b) == 0 && len(val) <= maxBufferSize {
-			PutByteBuffer(*b)
 			*b = GetByteBuffer()[:0]
 		} else {
 			PutByteBuffer(*b)