leak-detective: Make sure to actually call malloc() from calloc() hook

Newer versions of GCC are too "smart" and replace a call to malloc(X)
followed by a call to memset(0,X) with a call co calloc(), which obviously
results in an infinite loop when it does that in our own calloc()
implementation.  Using `volatile` for the variable storing the total size
prevents the optimization and we actually call malloc().

src/libstrongswan/utils/leak_detective.c

@@ -810,10 +810,11 @@ HOOK(void*, malloc, size_t bytes)
 HOOK(void*, calloc, size_t nmemb, size_t size)
 {
 	void *ptr;
+	volatile size_t total;
 
-	size *= nmemb;
+	total = nmemb * size;
-	ptr = malloc(size);
+	ptr = malloc(total);
-	memset(ptr, 0, size);
+	memset(ptr, 0, total);
 
 	return ptr;
 }