Note that Buffer.Unmarshal does NOT reset the destination protobuf.

When repeatedly Unmarshalling into the same destination, this can cause a memory leak.
PiperOrigin-RevId: 135304939

proto/decode.go

diff --git a/proto/decode.go b/proto/decode.go
index 04dcb88..a54a8cc 100644
--- a/proto/decode.go
+++ b/proto/decode.go
@@ -340,6 +340,8 @@
 // Buffer and places the decoded result in pb.  If the struct
 // underlying pb does not match the data in the buffer, the results can be
 // unpredictable.
+//
+// Unlike proto.Unmarshal, this does not reset pb before starting to unmarshal.
 func (p *Buffer) Unmarshal(pb Message) error {
 	// If the object can unmarshal itself, let it.
 	if u, ok := pb.(Unmarshaler); ok {