Change the callback function to use void**.

NOTE: This change breaks backwards-compatibility by default.
If you have old callback functions, you can define PB_OLD_CALLBACK_STYLE
to retain the old behaviour.

If you want to convert your old callbacks to new signature, you need
to do the following:

1) Change decode callback argument to   void **arg
      and encode callback argument to   void * const *arg.

2) Change any reference to arg into *arg.

The rationale for making the new behaviour the default is that it
simplifies the common case of "allocate some memory in decode callback".

Update issue 69
Status: FixedInGit

example/client.c

@@ -23,7 +23,7 @@
 #include "fileproto.pb.h"
 #include "common.h"
 
-bool printfile_callback(pb_istream_t *stream, const pb_field_t *field, void *arg)
+bool printfile_callback(pb_istream_t *stream, const pb_field_t *field, void **arg)
 {
     FileInfo fileinfo;