This PR fixes a critical issue where Coturn would permanently stop allocating ports after approximately 4 billion allocation/release operations due to uint32_t integer overflow.

Problem

The _turnports structure uses two uint32_t counters (low and high) to track port allocation state:

• low increments on each port allocation
• high increments on each port release

After ~4 billion operations, these counters would overflow. When high wraps around to a small value while low remains large, the condition tp->high <= tp->low in turnports_allocate() would become permanently true, preventing all future port allocations.

Solution

Changed the overflow-sensitive condition from:

if (tp->high <= tp->low) {
    return -1;  // Cannot allocate
}

To an overflow-safe condition using unsigned arithmetic:

if ((tp->high - tp->low) == 0) {
    return -1;  // Cannot allocate  
}

This works correctly in all scenarios:

• Normal case: high=2000, low=1000 → 2000-1000=1000 (ports available)
• No ports: high=low → high-low=0 (correctly identifies exhaustion)
• Overflow case: high=100, low=4294967000 → 100-4294967000=large number (correctly identifies available ports due to unsigned underflow)

Testing

• All existing tests continue to pass
• Added comprehensive validation covering normal operation, edge cases, and overflow scenarios
• Verified the fix handles the exact scenarios described in the issue

The server can now handle unlimited allocation/release cycles without the cumulative limit issue.

Fixes #1649.

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