fix marshal uninitialized variable warnings (#4114)

fix marshal uninitialized variable warnings (#4114)

GCC says:
../cpython/Python/marshal.c: In function ‘PyMarshal_WriteLongToFile’:
../cpython/Python/marshal.c:70:35: warning: ‘wf.ptr’ may be used uninitialized in this function [-Wmaybe-uninitialized]
                       else if ((p)->ptr != (p)->end) *(p)->ptr++ = (c); \
                                   ^~
../cpython/Python/marshal.c:70:47: warning: ‘wf.end’ may be used uninitialized in this function [-Wmaybe-uninitialized]
                       else if ((p)->ptr != (p)->end) *(p)->ptr++ = (c); \
                                               ^~
../cpython/Python/marshal.c:77:10: warning: ‘wf.str’ may be used uninitialized in this function [-Wmaybe-uninitialized]
     if (p->str == NULL)
         ~^~~~~

This isn't a real problem because if the file pointer is not NULL, the
string-related fields are never touched. But, it doesn't hurt to set the unused
fields to NULL.