fixed paramgrill wrong assert() conditions

and slightly refactored affected function.

Honestly, the formula calculating variance should get a second reviewing round,
it's not clear if it's correct.

programs/bench.h

@@ -45,7 +45,7 @@ typedef struct {
     size_t cSize;
     unsigned long long cSpeed;   /* bytes / sec */
     unsigned long long dSpeed;
-    size_t cMem;                 /* ? what is reported ? */
+    size_t cMem;                 /* memory usage during compression */
 } BMK_benchResult_t;
 
 VARIANT_ERROR_RESULT(BMK_benchResult_t, BMK_benchOutcome_t);