Fix GCC 4.9 build after rev.14893

Fix GCC 4.9 build after rev.14893

GCC 4.9 does not support some corner cases of C++11 syntax. In this case
when it is presented with default nulptr initialization of function
pointers it gets confused with 'pure virtual' initialization:

    error: invalid pure specifier (only '= 0' is allowed) before ';' token