Fix valgrind uninitialized memory errors

Fixed multiple uninitialized memory issues detected by valgrind:

1. SHA256 struct size mismatch: Changed from 64 bytes to 32 bytes to
   match the actual output of sha256() function (which produces 256
   bits = 32 bytes).

2. File tree chunk allocation: Fixed loop that wasn't initializing
   newly allocated chunks because num_chunks was updated before the
   initialization loop. Now saves old_num_chunks first.

3. Metadata server process_client_write: Initialized results[i].num_addrs
   to 0 before use to prevent reading uninitialized value.

4. Address struct initialization: Zero-initialize Address structs before
   setting fields to ensure the entire union is initialized (union
   contains either IPv4 or IPv6, leaving unused bytes uninitialized).

5. Fixed bug in IPv6 address creation: Changed incorrect is_ipv4=true
   to is_ipv4=false for IPv6 addresses.

6. Fixed all_chunk_servers_holding_chunk: Function was incrementing
   counter for all chunk servers instead of only those containing the
   hash, causing access to uninitialized array elements.

All valgrind errors resolved: ERROR SUMMARY: 0 errors from 0 contexts

src/file_tree.c

@@ -341,17 +341,18 @@ int file_tree_write(FileTree *ft, string path,
     uint64_t  last_chunk_index = (off + len - 1) / f->chunk_size;
 
     if (last_chunk_index >= f->num_chunks) {
+        uint64_t old_num_chunks = f->num_chunks;
         SHA256 *new_chunks = sys_malloc((last_chunk_index+1) * sizeof(SHA256));
         if (new_chunks == NULL)
             return FILETREE_NOMEM;
         if (f->chunks) {
             if (f->num_chunks > 0)
-                memcpy(new_chunks, f->chunks, f->num_chunks);
+                memcpy(new_chunks, f->chunks, f->num_chunks * sizeof(SHA256));
             sys_free(f->chunks);
         }
         f->chunks = new_chunks;
         f->num_chunks = last_chunk_index+1;
-        for (uint64_t i = f->num_chunks; i < last_chunk_index+1; i++)
+        for (uint64_t i = old_num_chunks; i < last_chunk_index+1; i++)
             memset(&f->chunks[i], 0, sizeof(SHA256));
     }