Remove the "buf = NULL" => skip requirement from pb_istream_t callbacks.

Rationale: it's easy to implement the callback wrong. Doing so introduces
io errors when unknown fields are present in the input. If code is not
tested with unknown fields, these bugs can remain hidden for long time.

Added a special case for the memory buffer stream, where it gives a small
speed benefit.

Added testcase for skipping fields with test_decode2 implementation.

Update issue 37
Status: FixedInGit

tests/test_decode2.c

@@ -59,13 +59,6 @@ bool callback(pb_istream_t *stream, uint8_t *buf, size_t count)
     FILE *file = (FILE*)stream->state;
     bool status;
     
-    if (buf == NULL)
-    {
-       /* Skipping data */
-        while (count-- && fgetc(file) != EOF);
-        return count == 0;
-    }
-    
     status = (fread(buf, 1, count, file) == count);
     
     if (feof(file))