AgentX error seen if Transceiver_info table contains bad string

Description

ERROR: Uncaught AgentX proto error is seen if TRANSCEIVER_INFO table has bad string in one of the fields.

Steps to reproduce the issue:

Bring up any DUT with master image or 2021x branch.
Inject bad string in any of the fields in TRANSCEIVER_DB. (redis-cli -n 6 ; HSET "TRANSCEIVER_INFO|Ethernet48" "hardware_rev" "\xff\xff")
Perform snmpwalk on transceiver MIB, snmpwalk -v2c -c public 127.0.0.1 iso.3.6.1.2.1.47.1.1.1.1.

Describe the results you received: snmpagent will restart with the below error:

ERR snmp#snmp-subagent [ax_interface] ERROR: Uncaught AgentX proto error! [b'\x01\x06\x10\x00\x00\x00\x00\x06\x00\x00/\xd4\x00\x00/\xd5\x00\x00\x00D\x08\x02\x00\x00\x00\x00\x00\x01\x00\x00\x00/\x00\x00\x00\x01\x00\x00\x00\x01\x00\x00\x00\x01\x00\x00\x00\x01\x00\x00\x00\x08;\x9a\xdb\xc6\x07\x02\x00\x00\x00\x00\x00\x01\x00\x00\x00/\x00\x00\x00\x01\x00\x00\x00\x01\x00\x00\x00\x01\x00\x00\x00\x01\x00\x00\x00\t']#012Traceback (most recent call last):#012  File "/usr/local/lib/python3.7/dist-packages/ax_interface/protocol.py", line 138, in data_received#012    response_pdu = pdu.make_response(self.mib_table)#012  File "/usr/local/lib/python3.7/dist-packages/ax_interface/pdu_implementations.py", line 289, in make_response#012    vr = lut.get_next(sr)#012  File "/usr/local/lib/python3.7/dist-packages/ax_interface/mib.py", line 359, in get_next#012    vr = self._get_nextvalue(parent_mib_entry, start_key)#012  File "/usr/local/lib/python3.7/dist-packages/ax_interface/mib.py", line 317, in _get_nextvalue#012    vr = ValueRepresentation.from_typecast(mib_entry.value_type, oid1, val1)#012  File "/usr/local/lib/python3.7/dist-packages/ax_interface/encodings.py", line 247, in from_typecast#012    _data = OctetString.from_string(data)#012  File "/usr/local/lib/python3.7/dist-packages/ax_interface/encodings.py", line 153, in from_string#012    _string = bytes(string, 'latin-1') if type(string) is str else string#012UnicodeEncodeError: 'latin-1' codec can't encode characters in position 0-1: ordinal not in range(256)

Describe the results you expected: AgentX should not crash due to bad string in Transceiver DB. Agentx should be stable and should validate strings that is read from DB table which is populated by external daemons.

Additional information you deem important (e.g. issue happens only occasionally):

**Output of `show version`:**

```
(paste your output here)
```

**Attach debug file `sudo generate_dump`:**

```
(paste your output here)
```

sonic-net / sonic-snmpagent

AgentX error seen if Transceiver_info table contains bad string #239