fix(jsbn): prevent modInverse hang for zero input

[Kr0emer]

Summary

This PR fixes a hang in jsbn when calling BigInteger#modInverse with a zero value.

When this == 0 and m is odd/non-zero (for example 0.modInverse(3)), bnModInverse() can enter an infinite loop in the while(v.isEven()) path, causing a potential DoS.

Root Cause

v is initialized as this.clone().
If this is zero, v stays zero forever, and right-shifting zero never changes it, so the loop does not terminate.

Fix

Add an early zero-input guard at the entry of bnModInverse():

• if(this.signum() == 0) return BigInteger.ZERO;

This keeps existing jsbn behavior for non-invertible cases (returning BigInteger.ZERO) and avoids introducing a breaking API change.

Tests

Added a regression test:

• tests/unit/jsbn.js
• included in tests/unit/index.js

The new test verifies BigInteger(0).modInverse(3) returns 0 and does not hang.

[sei-vsarvepalli]

@davidlehn - would like you to review when possible. The patch seems very reasonable.

[Kr0emer]

@davidlehn - would like you to review when possible. The patch seems very reasonable.

Thanks @sei-vsarvepalli. The previous PR was submitted to the wrong location. I've resubmitted the fix under the correct advisory page: GHSA-5m6q-g25r-mvwx

@davidlehn please review when you get a chance.

[davidlehn]

Fixed in 1.4.0.
See also: GHSA-5m6q-g25r-mvwx.