Initialize the mutexattr before use. (#3482)

As observed by @kkebo, this was missing from the implementation.

### Motivation:

Correct pthread API usage. 

### Modifications:

Initialize the pthread_mutexattr_t with pthread_mutexattr_init before
passed to pthread_mutex_init.

### Result:

Even though this lock API appears to be deprecated, we'll correct the
usage anyway. The non-deprecated NIOLock class doesn't have this
problem.

Author: 3405691582

Sources/NIOConcurrencyHelpers/lock.swift

@@ -57,7 +57,9 @@ public final class Lock {
         InitializeSRWLock(self.mutex)
         #elseif os(OpenBSD)
         var attr = pthread_mutexattr_t(bitPattern: 0)
-        let err = pthread_mutex_init(self.mutex, &attr)
+        var err = pthread_mutexattr_init(&attr)
+        precondition(err == 0, "\(#function) failed in pthread_mutexattr_init with error \(err)")
+        err = pthread_mutex_init(self.mutex, &attr)
         precondition(err == 0, "\(#function) failed in pthread_mutex with error \(err)")
         #elseif (compiler(<6.1) && !os(WASI)) || (compiler(>=6.1) && _runtime(_multithreaded))
         var attr = pthread_mutexattr_t()