http2: fixed the header accessor functions for the push callback

lib/http.h

@@ -176,7 +176,10 @@ struct HTTP {
   const uint8_t *upload_mem; /* points to a buffer to read from */
   size_t upload_len; /* size of the buffer 'upload_mem' points to */
   curl_off_t upload_left; /* number of bytes left to upload */
-  Curl_send_buffer *push_recvbuf; /* store incoming push headers */
+
+  char **push_headers;       /* allocated array */
+  size_t push_headers_used;  /* number of entries filled in */
+  size_t push_headers_alloc; /* number of entries allocated */
 #endif
 };