gob: make robust when decoding a struct with non-struct data.

The decoder was crashing when handling an rpc that expected
a struct but was delivered something else.  This diagnoses the
problem.  The other direction (expecting non-struct but getting
one) was already handled.

R=rsc
CC=golang-dev
https://golang.org/cl/2246041

src/pkg/gob/decode.go

@@ -843,12 +843,17 @@ func (dec *Decoder) compileDec(remoteId typeId, rt reflect.Type) (engine *decEng
 		return dec.compileSingle(remoteId, rt)
 	}
 	var wireStruct *structType
-	// Builtin types can come from global pool; the rest must be defined by the decoder
+	// Builtin types can come from global pool; the rest must be defined by the decoder.
+	// Also we know we're decoding a struct now, so the client must have sent one.
 	if t, ok := builtinIdToType[remoteId]; ok {
-		wireStruct = t.(*structType)
+		wireStruct, _ = t.(*structType)
 	} else {
 		wireStruct = dec.wireType[remoteId].structT
 	}
+	if wireStruct == nil {
+		return nil, os.ErrorString("gob: type mismatch in decoder: want struct type " +
+			rt.String() + "; got non-struct")
+	}
 	engine = new(decEngine)
 	engine.instr = make([]decInstr, len(wireStruct.field))
 	// Loop over the fields of the wire type.