fix: validate salt rounds to prevent hang on negative values

[abhu85]

Summary

• Add input validation to reject negative salt rounds with clear error message
• Fix error propagation bug where genSalt errors in hash() were silently ignored
• Add tests for negative rounds validation (sync, async, and promise APIs)

Problem

Calling bcrypt.hash('password', -1, callback) caused the process to hang indefinitely (issue #1218).

Root cause: Negative rounds values were passed to the C++ layer, which clamped them to 31 (the maximum valid value). This resulted in 2^31 = 2,147,483,648 iterations - effectively infinite computation time.

Solution

Validate that rounds must be non-negative in both genSalt() and genSaltSync(). The error is thrown/returned consistently across all APIs:

• Sync: throws Error('rounds must be a positive integer')
• Async callback: passes error to callback
• Promise: rejects with the error

Also fixed a pre-existing bug where errors from genSalt() inside hash() were not propagated to the callback - the callback was called with undefined salt, causing a TypeError.

Test Plan

• Added salt_rounds_is_negative test in sync.test.js
• Added hash_rounds_is_negative test in sync.test.js
• Added salt_rounds_is_negative test in async.test.js
• Added hash_rounds_is_negative test in async.test.js
• Added salt_rounds_is_negative test in promise.test.js
• Added hash_rounds_is_negative test in promise.test.js
• All existing tests pass (81 total)

Fixes #1218