go/neo/proto/msgpack: Fix 'types.Struct' decoding

In NEO/go protocol, we describe some parameters that can
be send via NEO packages with structures. On the wire all of
these structures are encoded as msgpack arrays [1]. Msgpack
arrays can have 1 or more bytes as a header [2]. Therefore
it's better to use "ReadArrayHeaderBytes" than using

  >>> data = data[1:]

which fails in case we have array with 3 or 5 header bytes.

[1] One could also declare a protocol where these parameters aren't
    send as msgpack arrays but as msgpack maps. See here for the
    msgpack map specification:

    https://github.com/msgpack/msgpack/blob/9aa092d6ca81/spec.md?plain=1#L338

[2] https://github.com/msgpack/msgpack/blob/9aa092d6ca81/spec.md?plain=1#L315

go/neo/proto/protogen.go

@@ -635,6 +635,10 @@ type OverflowCheck struct {
 
 	// stack operated by {Push,Pop}Checked
 	checkedStk []bool
+
+	// whether any 'goto overflow' has been emitted for
+	// current decoder
+	gotoEmitted bool
 }
 
 // push/pop checked state
@@ -819,6 +823,8 @@ func (d *decoderCommon) overflowCheck() {
 			lendata = "uint64(" + lendata + ")"
 		}
 		d.bufDone.emit("if %s < %v { goto overflow }", lendata, &d.overflow.checkSize)
+		// Flag that we already committed a 'goto overflow' statement in current decoder
+		d.overflow.gotoEmitted = true
 
 		// if size for overflow check was only numeric - just
 		// accumulate it at generation time
@@ -901,7 +907,7 @@ func (d *decoderCommon) generatedCode() string {
 
 	// `goto overflow` is not used only for empty structs
 	// NOTE for >0 check actual X in StdSizes{X} does not particularly matter
-	if (&types.StdSizes{8, 8}).Sizeof(d.typ) > 0 || d.enc != 'N' {
+	if ((&types.StdSizes{8, 8}).Sizeof(d.typ) > 0 || d.enc != 'N') && d.overflow.gotoEmitted {
 		code.emit("\noverflow:")
 		code.emit("return 0, ErrDecodeOverflow")
 	}
@@ -1735,8 +1741,22 @@ func (d *decoderM) genStructHead(path string, typ *types.Struct, userType types.
 	d.emit("return 0, &mstructDecodeError{%q, op, opOk}", d.pathName(path))
 	d.emit("}")
 
-	d.n += 1
-	d.overflow.Add(1)
+	d.resetPos()
+
+	// we are going to go into msgp - flush previously queued
+	// overflow checks; put place for next overflow check after
+	// msgp is done.
+	d.overflowCheck()
+	defer d.overflowCheck()
+
+	d.emit("{")
+	d.emit("l, tail, err := msgp.ReadArrayHeaderBytes(data)")
+	d.emit("if err != nil {")
+	d.emit(fmt.Sprintf("return 0, mdecodeErr(%q, err)", d.pathName(path)))
+	d.emit("}")
+	d.emit("data = tail")
+	d.emit("%v += uint64(l)", d.var_("nread"))
+	d.emit("}")
 }