Prevent negative values in btc scripts

[jrwbabylonlab]

In btc-staking-ts, there are multiple functions that claim to validate whether values are negative but do so incorrectly.

In addition, the ObservableStaking class does not validate whether the btcActivationHeight field is a negative number.

A user or dApp would not be prevented from accidentally supplying a negative number for these fields, which could lead to unintended behavior.
In the case of StakingScripts, this value would be compiled into the Bitcoin script and passed to the OP_CHECKSEQUENCEVERIFY opcode, which could lead to errors upon execution.
Recommendations
Fix the checks to prevent negative values.

Examples

// check that maximum value for staking time is not greater than uint16 and
      above 0
  if (this.stakingTimeLock == 0 || this.stakingTimeLock > 65535) {
      return false;
}
  // check that maximum value for unbonding time is not greater than uint16 and
      above 0
  if (this.unbondingTimeLock == 0 || this.unbondingTimeLock > 65535) {
      return false;
}

[jrwbabylonlab]

#68