Fix memory leak in pthread init functions on failure

[rlefko]

Summary

This PR fixes a memory leak in the debug implementation of pthread initialization functions when pthread_mutex_init() or pthread_cond_init() fails.

Problem

In the debug implementation (used when DEBUGLEVEL >= 1), the ZSTD_pthread_mutex_init() and ZSTD_pthread_cond_init() functions allocate memory using ZSTD_malloc() but fail to free it if the subsequent pthread initialization calls fail. This leads to memory leaks in error conditions.

The affected functions are:

• ZSTD_pthread_mutex_init() at lib/common/threading.c:146
• ZSTD_pthread_cond_init() at lib/common/threading.c:167

Solution

The fix adds proper error handling to free the allocated memory when pthread initialization fails:

1. Store the return value of pthread_mutex_init/pthread_cond_init
2. If initialization fails (non-zero return), free the allocated memory and set the pointer to NULL
3. Return the error code

Impact

This fix is particularly important for:

• Long-running applications where repeated initialization failures could accumulate memory leaks
• Resource-constrained environments where pthread initialization might fail due to system limits
• Ensuring proper resource cleanup in all error paths

Testing

• All existing tests pass (make check completed successfully)
• Pool tests specifically verified (make poolTests && ./poolTests passed)
• The fix only affects error paths and maintains the same interface behavior

Changes

• Modified lib/common/threading.c to add proper cleanup on pthread initialization failure

This is a defensive programming improvement that ensures no resources are leaked even in uncommon error scenarios.

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[Cyan4973]

This is a pretty straightforward fix.
Thanks @rlefko !