Unbond with less than 1 nano atom

[fedekunze]

Describe the bug

Either something with the precision is wrong on the SDK or we're not calculating shares correctly.

The minimum allowed precision is a utoms (nano atoms or 1e-7 atoms).

To Reproduce

Undelegate 5e-12 stake which translates to "shares_amount": "0.000050000000000000"

Expected behavior

Not allow to undelegate less than 1 nano atom

Screenshots

Additional context

Transaction:

{
    "height": "4184",
    "txhash": "90BC800E95C9C35BAD3F8180DD4A3B3F319C46AB2F17006E2676EBCAB48A3323",
    "data": "DAimgpnkBRDgof6tAQ==",
    "log": "[{\"msg_index\":\"0\",\"success\":true,\"log\":\"\"}]",
    "gas_wanted": "500000",
    "gas_used": "101024",
    "tags": [
      {
        "key": "action",
        "value": "begin_unbonding"
      },
      {
        "key": "delegator",
        "value": "cosmos1h0k4vl3kfjurxfemnlhr684px8u6q3anz2f8ct"
      },
      {
        "key": "source-validator",
        "value": "cosmosvaloper1h0k4vl3kfjurxfemnlhr684px8u6q3an87aj5c"
      },
      {
        "key": "end-time",
        "value": "2019-03-11T11:06:14Z"
      }
    ],
    "tx": {
      "type": "auth/StdTx",
      "value": {
        "msg": [
          {
            "type": "cosmos-sdk/Undelegate",
            "value": {
              "delegator_addr": "cosmos1h0k4vl3kfjurxfemnlhr684px8u6q3anz2f8ct",
              "validator_addr": "cosmosvaloper1h0k4vl3kfjurxfemnlhr684px8u6q3an87aj5c",
              "shares_amount": "0.000050000000000000"
            }
          }
        ],
        "fee": {
          "amount": null,
          "gas": "500000"
        },
        "signatures": [
          {
            "pub_key": {
              "type": "tendermint/PubKeySecp256k1",
              "value": "Aiuxl8dL1b/EIFdy7+W4Q8acxbGmr/1OdIIYuqOdmV+A"
            },
            "signature": "D/SjZSZi8+yBw4iO0hQ5uEfpeOe/5u3QRoGxlHNrE0VZFTeUeOvbhQKuHLnC5U6ma3K4FtdhL0RQMMVoRWo2Tw=="
          }
        ],
        "memo": "Sent via Cosmos UI 🚀"
      }
    }
  }

[fedekunze]

the validator had delegator_shares: 99999999.999950000000000000