fix: const-correct delta.c and use unread variable in test for cppcheck

src/shared/delta.c

@@ -422,7 +422,7 @@ void* delta_apply(const void* old_data, uint64_t old_size, const Delta* delta,
     return NULL;
 
   uint8_t* out = (uint8_t*)output;
-  uint8_t* old = (uint8_t*)old_data;
+  const uint8_t* old = (const uint8_t*)old_data;
   uint64_t out_pos = 0;
 
   for (uint32_t i = 0; i < delta->instruction_count; i++) {

tests/test_delta.c

@@ -307,6 +307,7 @@ static void test_large_file_delta() {
     else
       match_count++;
   }
+  EXPECT_TRUE(total_literal > 0);
   EXPECT_TRUE(match_count > 0);
   EXPECT_TRUE(delta->delta_size < new_size / 2);