Regression since 1.1.1: discard when reading from sequence after error

Hello, I noted a regression when using rmp_serde 1.2.0 and over, when trying to read from a deserializer that had an error

consider the following script:

```rust
use std::fmt;

use rmp_serde;
use serde::Deserialize;
use serde::de::{Deserializer, Visitor};

// a sample struct that expects an array with one i8 at the end
// to simulate error recovery
struct ContinueTillI8(i8);

impl<'de> Deserialize<'de> for ContinueTillI8 {
fn deserialize(deserializer: D) -> Result
where
D: Deserializer<'de>,
{
struct ContinueTillI8Visitor;

impl<'de> Visitor<'de> for ContinueTillI8Visitor {
type Value = ContinueTillI8;

fn expecting(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
formatter.write_str("array that has one i8 at the end")
}

fn visit_seq(self, mut seq: A) -> Result
where
A: serde::de::SeqAccess<'de>, {
loop {
match seq.next_element() {
Ok(Some(i)) => {
return Ok(ContinueTillI8(i));
}
Err(e) => {
println!("discraded element with err: {:?}", e);
}
Ok(None) => {
return Err(serde::de::Error::custom("No i8 found"));
}
}
}
}
}

this is likely due to rmp_serde not consuming the `FixStr(2)`'s actual value after detecting it is not an i8, leaving the deserailizer in an invalid state