This bug can only occur when all of the following apply:

1) Message contains extension fields and
2) the extension field is of callback type and
3) the pb_extension_t structure is defined, but the pointer to
   pb_callback_t is left as NULL.

The correct behaviour in this case is that the field will be skipped.

Further, for most compilers atleast and probably for all compilers, the
bug will only occur if PB_OLD_CALLBACK_STYLE compilation option is specified.