Slice struct with all "omitempty" fields set to zero value ​​will generate wrong data

[ljfuyuan]

look at this sample below:

type Sample struct {
	K        uint32 `msg:"k,omitempty"`
        V        uint32 `msg:"v,omitempty"`
}

type Samples []Sample

and now we construct a set of test cases:

s := Samples{
		{1, 1},
		{0, 0},
		{3, 3},
	}

and the generated code is:

	for za0001 := range z {
		// check for omitted fields
		zb0001Len := uint32(2)
		var zb0001Mask uint8 /* 2 bits */
		_ = zb0001Mask
		if z[za0001].K == 0 {
			zb0001Len--
			zb0001Mask |= 0x1
		}
		if z[za0001].V == 0 {
			zb0001Len--
			zb0001Mask |= 0x2
		}
		// variable map header, size zb0001Len
		o = append(o, 0x80|uint8(zb0001Len))
		if zb0001Len == 0 {
			return     //  <---- !!! When looping to the second element, wrong data will be generated due to return. Continue may be used here. 

		}
...... ignore

[klauspost]

Please try #377