fix(stacking): fix "Assertion failed" error when calling .stack and .canStack

[pradel]

Description

When integrating the new @stacks/stacking npm package I am having an issue with the client.stack and the client.canStack methods, both are throwing [Error: Assertion failed].

After some debugging, it looks like the error is coming from this call https://github.com/blockstack/stacks.js/blob/a6b7da0586caa893c7cdf2ef4a3011de3248afe8/packages/stacking/src/index.ts#L319.
hashMode returned by const { hashMode, data } = decodeBtcAddress(poxAddress); is 0 and calling bn.js new BN(hashMode, 10).toBuffer() to get the buffer on 0 is throwing an error

Related issue #898.

Example:

Following the stacking tutorial and doing a simple operation like this:

const stackingClient = new StackingClient(
  "STRW09277YXKS1GZFT7QQSQ7GNWS2M4S3CG5V6ZT",
  new StacksTestnet()
);

const stackingEligibility = await stackingClient.canStack({
  poxAddress: "tb1qa96uwhml8rxfees4ggn98z55xlmdxnrxhe60n2jl29vyg5txcnts36xrr2",
  cycles: 1,
});

For details refer to issue #123

Type of Change

• New feature
• Bug fix
• API reference/documentation update
• Other

Does this introduce a breaking change?

No

Are documentation updates required?

No

Testing information

Provide context on how tests should be performed.

1. Is testing required for this change?

I tested to manually patch the file in my project and the error was fixed with this change.

2. If it’s a bug fix, list steps to reproduce the bug

const stackingClient = new StackingClient(
  "STRW09277YXKS1GZFT7QQSQ7GNWS2M4S3CG5V6ZT",
  new StacksTestnet()
);

const stackingEligibility = await stackingClient.canStack({
  poxAddress: "tb1qa96uwhml8rxfees4ggn98z55xlmdxnrxhe60n2jl29vyg5txcnts36xrr2",
  cycles: 1,
});

Checklist

• Code is commented where needed
• Unit test coverage for new or modified code paths
• npm run test passes
• Changelog is updated
• Tag 1 of @yknl or @zone117x for review

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All committers have signed the CLA.

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[codecov]

Codecov Report

Merging #925 (cfac222) into master (788006e) will decrease coverage by 0.20%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #925      +/-   ##
==========================================
- Coverage   77.10%   76.89%   -0.21%     
==========================================
  Files          72       72              
  Lines        4385     4488     +103     
  Branches      747      784      +37     
==========================================
+ Hits         3381     3451      +70     
- Misses        969     1000      +31     
- Partials       35       37       +2     

Impacted Files	Coverage Δ
src/index.ts	78.30% <0.00%> (-21.70%)	⬇️
src/utils.ts	52.52% <0.00%> (-1.33%)	⬇️
src/constants.ts	100.00% <0.00%> (ø)

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