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/shared/protocol.h

@@ -20,14 +20,13 @@ enum NET_STATUS {
 
 void io_set_fds(int read_fd, int write_fd);
 void io_set_bwlimit(unsigned long long bytes_per_sec);
-typedef struct ssl_st SSL;
 void io_set_ssl(SSL* ssl);
-bool send_n_data(int file_descriptor, void* data, size_t data_size);
+bool send_n_data(int file_descriptor, const void* data, size_t data_size);
 bool receive_n_data(int file_descriptor, void* data, size_t data_size);
 
-bool send_str(int file_descriptor, char* data);
+bool send_str(int file_descriptor, const char* data);
 char* receive_str(int file_descriptor);
-bool send_data(int file_descriptor, Data* data);
+bool send_data(int file_descriptor, const Data* data);
 Data* receive_data(int file_descriptor);
 bool send_int(int file_descriptor, int data);
 bool receive_int(int file_descriptor, int* data);