Added checking of arguments to COST_ADD and COST_MULT

These functions don't work with negative values and should never be
called with negative values. Added an assert to ensure this will
not happen.

sql/optimizer_costs.h

@@ -179,8 +179,18 @@ static inline double cache_hit_ratio(uint ratio)
 
 #define COST_MAX (DBL_MAX * (1.0 - DBL_EPSILON))
 
-#define COST_ADD(c,d) (COST_MAX - (d) > (c) ? (c) + (d) : COST_MAX)
+static inline double COST_ADD(double c, double d)
+{
+  DBUG_ASSERT(c >= 0);
+  DBUG_ASSERT(d >= 0);
+  return (COST_MAX - (d) > (c) ? (c) + (d) : COST_MAX);
+}
 
-#define COST_MULT(c,f) (COST_MAX / (f) > (c) ? (c) * (f) : COST_MAX)
+static inline double COST_MULT(double c, double f)
+{
+  DBUG_ASSERT(c >= 0);
+  DBUG_ASSERT(f >= 0);
+  return (COST_MAX / (f) > (c) ? (c) * (f) : COST_MAX);
+}
 
 #endif /* OPTIMIZER_COSTS_INCLUDED */