Jeff Pohlmeyer presented a *multi_socket()-using program that exposed a

problem with it (SIGSEGV-style). It clearly showed that the existing
  socket-state and state-difference function wasn't good enough so I rewrote
  it and could then re-run Jeff's program without any crash. The previous
  version clearly could miss to tell the application when a handle changed
  from using one socket to using another.

  While I was at it (as I could use this as a means to track this problem
  down), I've now added a 'magic' number to the easy handle struct that is
  inited at curl_easy_init() time and cleared at curl_easy_cleanup() time that
  we can use internally to detect that an easy handle seems to be fine, or at
  least not closed or freed (freeing in debug builds fill the area with 0x13
  bytes but in normal builds we can of course not assume any particular data
  in the freed areas).

lib/url.c

@@ -214,6 +214,8 @@ CURLcode Curl_close(struct SessionHandle *data)
 {
   struct Curl_multi *m = data->multi;
 
+  data->magic = 0; /* force a clear */
+
   if(m)
     /* This handle is still part of a multi handle, take care of this first
        and detach this handle from there. */
@@ -374,6 +376,8 @@ CURLcode Curl_open(struct SessionHandle **curl)
     /* this is a very serious error */
     return CURLE_OUT_OF_MEMORY;
 
+  data->magic = CURLEASY_MAGIC_NUMBER;
+
 #ifdef USE_ARES
   if(ARES_SUCCESS != ares_init(&data->state.areschannel)) {
     free(data);