Bug 1053: Fix crash when download ends prematurely.

Bug 1053: Fix crash when download ends prematurely.

Call stacks reported by valgrind:

==14702==    at 0x80DD791: read_from_socket (socket.c:945)
==14702==    by 0x8104D0C: read_more_http_data (http.c:1180)
==14702==    by 0x81052FE: read_http_data (http.c:1388)
==14702==    by 0x80DD69B: read_select (socket.c:910)
==14702==    by 0x80D27AA: select_loop (select.c:307)
==14702==    by 0x80D1ADE: main (main.c:358)
==14702==  Address 0x4F4E598 is 56 bytes inside a block of size 81 free'd
==14702==    at 0x402210F: free (vg_replace_malloc.c:233)
==14702==    by 0x812BED8: debug_mem_free (memdebug.c:484)
==14702==    by 0x80D7C82: done_connection (connection.c:479)
==14702==    by 0x80D8A44: abort_connection (connection.c:769)
==14702==    by 0x80D99CE: cancel_download (connection.c:1053)
==14702==    by 0x8110EB6: abort_download (download.c:143)
==14702==    by 0x81115BC: download_data_store (download.c:337)
==14702==    by 0x8111AFB: download_data (download.c:446)
==14702==    by 0x80D7B33: notify_connection_callbacks (connection.c:458)
==14702==    by 0x80D781E: set_connection_state (connection.c:388)
==14702==    by 0x80D7132: set_connection_socket_state (connection.c:234)
==14702==    by 0x80DD78D: read_from_socket (socket.c:943)

read_from_socket() attempted to read socket->fd in order to set
handlers on it, but the socket had already been freed.  Incidentally,
socket->fd was -1, which would have resulted in an assertion failure
if valgrind hadn't caught the bug first.

To fix this, add a list of weak references to sockets.
read_from_socket() registers a weak reference on entry and unregisters
it before exit.  done_socket() breaks any weak references to the
specified socket.  read_from_socket() then checks whether the weak
reference was broken, and doesn't access the socket any more if so.