Fix memory leaks in password encoding error paths

[Sn0wyDay]

• Have you signed the Eclipse Contributor Agreement, using the same email address as you used in your commits?
• Do each of your commits have a "Signed-off-by" line, with the correct email address?
• If you are contributing a new feature, is your work based off the develop branch? (Not applicable: this is a bug fix.)
• If you are contributing a bugfix, is your work based off the fixes branch?
• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you successfully run make test with your changes locally? (The full suite could not run locally because GTest, CUnit, and microsocks were unavailable; the focused password tests and sanitizer checks passed.)

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pw__encode_sha512() and pw__encode_sha512_pbkdf2() leave their temporary Base64 buffers allocated on some allocation-failure paths. The SHA-512 path also leaves the encoded salt allocated when encoding the password hash fails.

Free the temporary buffers before returning from those paths. Successful password encoding is unchanged.

Validation:

• Built the project with CMake on macOS.
• Passed test/apps/passwd/passwd-args.py, passwd-stdout.py, and passwd-changes.py.
• Exercised all three affected failure paths with Mosquitto's memory limit and memory tracking. Before this change, cleanup left 96, 192, and 192 bytes allocated on this platform; after this change, all three paths leave zero bytes allocated while preserving their return codes.
• Repeated the failure-path checks in an AddressSanitizer and UndefinedBehaviorSanitizer build.

This bug fix is based on the fixes branch.