Fix integer overflow in comparison function

The comparison function used in qsort() returns *a - *b, which can cause
integer overflow when the difference exceeds the range of int. This
leads to incorrect sorting results when execution time measurements
exceed approximately 2.1 billion cycles, which is possible in certain
cases.

Fix this by returning -1, 0, or 1 explicitly based on the comparison of
*a and *b, avoiding overflow while maintaining correct sorting behavior.

Author: BennyWang1007

dudect/fixture.c

@@ -69,7 +69,9 @@ static int64_t percentile(const int64_t *a_sorted, double which, size_t size)
 
 static int cmp(const int64_t *a, const int64_t *b)
 {
-    return *a - *b;
+    if (*a == *b)
+        return 0;
+    return (*a - *b) > 0 ? 1 : -1;
 }
 
 /* This function is used to set different thresholds for cropping measurements.