fix:(encoder) insufficient buffer size check for OP_i32

[AsterDY]

• since Go runtime always allocates memory of JIT stack _Stack right after that of output buffer buf:
  

• _OP_i32 has a boundary check of size 11, but in native-C code, when the integer is bigger than 99999999, the program will use vmovdqu %xmm0, (%rdi) instruction to write a 16-byte chars into buf and cross the bound of it
  

• once buffer has no space more than 16B, out-of-range memory writing can cause the corruption of _Stack, which finally causes abnormal behaviors of the program
  

Here is reproducible codes:

func TestEncoderMemoryCorruption(t *testing.T) {
    var m = map[string]interface{}{
        "1": map[string]interface{} {
            `"`+strings.Repeat("a", _MaxBuffer - 38)+`"`: "b",
            "1": map[string]int32{
                "b": 1658219785,
            },
        },
    }
    out, err := Encode(m, SortMapKeys)
    if err != nil {
        t.Fatal(err)
    }
    println(len(out))
    if err := json.Unmarshal(out, &m); err != nil {
        t.Fatal(err)
    }
}

-------output----------
--- FAIL: TestEncoderMemoryCorruption (0.02s)
    /Users/admin/Desktop/kitex/sonic3/encoder/assembler_test.go:50: invalid character '\x00' in string literal
FAIL