fix: prevent integer overflow in provide_at_least()

When available_clones.size() > n, the subtraction (n - available_clones.size())
caused unsigned arithmetic underflow, wrapping to INT_MAX (2^31-1). This made
the code try to allocate 2 billion clones, causing catastrophic memory growth.

The fix checks if we already have enough available clones before performing
the arithmetic. This prevents the underflow and keeps memory usage reasonable.

Before: decay_of_LD_highd.py would hit 12GB after 5 generations
After: Completes all 250 generations with <1GB memory

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <[email protected]>

Author: corneliusroemer

src/haploid_highd.cpp

@@ -212,6 +212,10 @@ int haploid_highd::free_mem() {
 int haploid_highd::provide_at_least(int n) {
 	//calculate the number of clones that need to be newly allocated. Allow for some slack
 	//to avoid calling this too often
+	// BUG FIX: Check if we already have enough clones to avoid unsigned arithmetic underflow
+	if (available_clones.size() >= n) {
+		return 0;  // We already have enough clones available
+	}
 	int needed_gts = n - available_clones.size() + 100 + 0.1 * population.size();
 
 	//allocate at the necessary memory