fix: const-correct protocol send functions, remove duplicate typedef, null check

- send_n_data: void* data → const void* data
- send_str: char* data → const char* data
- send_data: Data* data → const Data* data
- Remove duplicate typedef struct ssl_st SSL in protocol.h
- Add NULL check after malloc in directory_scanner_create

src/client/scanner.c

@@ -17,6 +17,8 @@ DirectoryScanner* directory_scanner_create(char* root_directory, bool use_metada
                                            int include_count, unsigned long long max_size,
                                            unsigned long long min_size) {
   DirectoryScanner* scanner = malloc(sizeof(DirectoryScanner));
+  if (scanner == NULL)
+    return NULL;
   scanner->directories = queue_create(100, free);
   scanner->current_dir = NULL;
   scanner->current_path = NULL;