Deadlock due to Recursive Mutex Locking

[APshenkin]

Describe the bug
The library experiences a deadlock when initialization fails due to insufficient lockable memory (mlock) on memory-constrained systems, particularly Android devices with a 64KB mlock limit. When the system has insufficient lockable memory available and memguard attempts to initialize, a deadlock occurs due to recursive mutex locking in the getOrCreateKey function.

Root Cause

1. The getOrCreateKey function acquires a lock on the key mutex

   memguard/core/enclave.go

   Line 14 in b28f395

   keyMtx.Lock()

2. When memory locking fails (due to mlock limits), an error occurs

   memguard/core/buffer.go

   Line 74 in b28f395

   Panic(err)

3. The error handling calls a Panic function
4. The Panic function attempts to acquire the same key mutex that is already held

   memguard/core/exit.go

   Line 22 in b28f395

   keyMtx.Lock()

5. This results in a deadlock, causing the application to freeze

To Reproduce
Steps to reproduce the behaviour:

1. Set up an Android environment (or create env in some other way with limited mlock)
2. Consume some of the available lockable memory
3. Initialize memguard library for the first time (e.g. call memguard.NewEnclave())
4. Observe application freeze during initialization

Expected behaviour
Getting crash or error

System (please complete the following information):

• OS and Kernel Versions: Android (64KB mlock limit)
• Memguard Version:
• Go Version: 1.24.0

[APshenkin]

I tried to fix this in this PR #174, but probably there are better way to do so