Tests: test_index: Make it clear that my_alloc() has no integer overflows

liblzma guarantees that the product of the allocation size arguments
will fit in size_t.

Putting the pre-increment in the if-statement was clearly wrong
although in practice it didn't matter here as the function is
called only a couple of times.

(cherry picked from commit 7f865577a6)

tests/test_index.c

@@ -1282,10 +1282,13 @@ my_alloc(void *opaque, size_t a, size_t b)
 {
 	(void)opaque;
 
+	assert_true(SIZE_MAX / a >= b);
+
 	static unsigned count = 0;
-	if (++count > 2)
+	if (count >= 2)
 		return NULL;
 
+	++count;
 	return malloc(a * b);
 }