Message Length incorrectly set on some PDUs

The log of our MT is available at request

```
At least i see message length is wrong:     Message length: 3. According to
you logs it is much longer.

Short Message Peer to Peer, Command: Submit_sm, Seq: 28722, Len: 944
    Length: 944
    Operation: Submit_sm (0x00000004)
    Sequence #: 28722
    Service type: (Default)
    Type of number (originator): Unknown (0x00)
    Numbering plan indicator (originator): Unknown (0x00)
    Originator address: 254731222093
    Type of number (recipient): International (0x01)
    Numbering plan indicator (recipient): ISDN (E163/E164) (0x01)
    Recipient address: 254787100382
    .... ..00 = Messaging mode: Default SMSC mode (0x00)
    ..00 00.. = Message type: Default message type (0x00)
    00.. .... = GSM features: No specific features selected (0x00)
    Protocol id.: 0x00
    Priority level: GSM: None      ANSI-136: Bulk         IS-95: Normal
(0x00)
    Scheduled delivery time: Immediate delivery
    Validity period: SMSC default validity period
    .... ..01 = Delivery receipt: Delivery receipt requested (for success
or failure) (0x01)
    .... 00.. = Message type: No recipient SME acknowledgement requested
(0x00)
    ...0 .... = Intermediate notif: No intermediate notification requested
(0x00)
    .... ...0 = Replace: Don't replace (0x00)
    Data coding: 0x08
        SMPP Data Coding Scheme: UCS2 (ISO/IEC-10646) (0x08)
        GSM SMS Data Coding
        0000 .... = DCS Coding Group for SMS: SMS DCS: General Data Coding
indication - Uncompressed text, no message class (0x00)
        ..0. .... = DCS Text compression: Uncompressed text
        ...0 .... = DCS Class present: No message class
        .... 10.. = DCS Character set: UCS-2 (16-bit) data (0x02)
        GSM CBS Data Coding
        0000 .... = DCS Coding Group for CBS: CBS DCS: Language using the
GSM 7-bit default alphabet (0x00)
        ..00 1000 = DCS CBS Message language: Portuguese (0x08)
    Predefined message: 0
    Message length: 3
    Message
    Optional parameters
        Optional parameter: 0x4100 (0x4100)
            Tag: 0x4100
            Length: 8192
[Malformed Packet: SMPP]
    [Expert Info (Error/Malformed): Malformed Packet (Exception occurred)]
        [Message: Malformed Packet (Exception occurred)]
        [Severity level: Error]
        [Group: Malformed]
```