Fix user-after-free in network code

[Mizarka]

Current code frees the C handle, which is used afterwards in a call to free_mp_queue (C->free_block_queue);, resulting in a classic use-after-free for which GCC rightfully complains:

net/net-msg-buffers.c:307:23: warning: pointer ‘C’ may be used after ‘free’ [-Wuse-after-free]
  307 |   C->free_block_queue = NULL;
      |                       ^
net/net-msg-buffers.c:294:3: note: call to ‘free’ here
  294 |   free (C);
      |   ^~~~~~~~
net/net-msg-buffers.c:306:3: warning: pointer ‘C’ may be used after ‘free’ [-Wuse-after-free]
  306 |   free_mp_queue (C->free_block_queue);
      |   ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
net/net-msg-buffers.c:294:3: note: call to ‘free’ here
  294 |   free (C);
      |   ^~~~~~~~

This PR fixes it by simply freeing at the very tail end of the function.