Add consistency check to ssh_set_callbacks

include/libssh/callbacks.h

@@ -244,7 +244,7 @@ typedef struct ssh_packet_callbacks_struct *ssh_packet_callbacks;
  *
  * @param  cb           The callback itself.
  *
- * @return 0 on success, < 0 on error.
+ * @return SSH_OK on success, SSH_ERROR on error.
  */
 LIBSSH_API int ssh_set_callbacks(ssh_session session, ssh_callbacks cb);
 

include/libssh/legacy.h

@@ -22,6 +22,8 @@
 /* Since libssh.h includes legacy.h, it's important that libssh.h is included
  * first. we don't define LEGACY_H now because we want it to be defined when
  * included from libssh.h
+ * All function calls declared in this header are deprecated and meant to be
+ * removed in future.
  */
 
 #include "libssh/libssh.h"

libssh/callbacks.c

@@ -28,10 +28,16 @@
 
 int ssh_set_callbacks(ssh_session session, ssh_callbacks cb) {
   if (session == NULL || cb == NULL) {
-    return -1;
+    return SSH_ERROR;
+  }
+  enter_function();
+  if(cb->size <= 0 || cb->size > 1024 * sizeof(void *)){
+  	ssh_set_error(session,SSH_FATAL,
+  			"Invalid callback passed in (badly initialized)");
+  	leave_function();
+  	return SSH_ERROR;
   }
-
   session->callbacks = cb;
-
+  leave_function();
   return 0;
 }