multi.c: fix segfault

#define GOOD_MULTI_HANDLE(x) \

  ((x) && (((struct Curl_multi *)(x))->type == CURL_MULTI_HANDLE))

#define GOOD_EASY_HANDLE(x) \

 (((struct SessionHandle *)(x))->magic == CURLEASY_MAGIC_NUMBER)

  ((x) && (((struct SessionHandle *)(x))->magic == CURLEASY_MAGIC_NUMBER))

/* This is the struct known as CURLM on the outside */

struct Curl_multi {