http2: initial implementation of the push callback

lib/urldata.h

@@ -198,6 +198,8 @@
 #define HEADERSIZE 256
 
 #define CURLEASY_MAGIC_NUMBER 0xc0dedbadU
+#define GOOD_EASY_HANDLE(x) \
+  ((x) && (((struct SessionHandle *)(x))->magic == CURLEASY_MAGIC_NUMBER))
 
 /* Some convenience macros to get the larger/smaller value out of two given.
    We prefix with CURL to prevent name collisions. */