Release the DRBG mutex when generation fails

[EdouardMALOT]

Summary

_nx_crypto_drbg() takes NX_CRYPTO_DRBG_MUTEX_GET at the top, but returns early when _nx_crypto_drbg_generate() fails, so the mutex is never released.

Both macros are empty by default and nothing in the repository defines them, so a default build is unaffected. Ports that do define them get a permanent deadlock instead of a returned error.

Found while working on #399.

Changes

• crypto_libraries/src/nx_crypto_drbg.c — move NX_CRYPTO_DRBG_MUTEX_PUT to just after _nx_crypto_drbg_generate(), before the status check. This also leaves the masking of the extra bits outside the critical section, which is fine since it only touches the caller's buffer.

I left the ignored _nx_crypto_drbg_initialize() return value on the line above alone; that is a separate question, and happy to look at it if you would like it in the same PR.

Test plan

• No behaviour change in a default build, where both macros expand to nothing
• Compiles clean under -Wall -Wextra -Wconversion
• Expanded both macros to function calls and checked there is exactly one get and one put, with the put now unconditional
• Not exercised at runtime: I have no port here that defines the mutex macros, so the deadlock itself is reasoned from the code rather than reproduced