Fix Windows warnings

[Smit-create]

Related issue: rgommers/scipy#120

This PR aims to fix the following warning:

[516/1557] Compiling C object scipy/stats/_unuran/unuran_wrapper.cp39-win_amd64.pyd.p/.._..__lib_unuran_unuran_src_methods_mvtdr.c.obj
In file included from ../scipy/_lib/unuran/unuran/src/methods/mvtdr.c:330:
In function '_unur_mvtdr_etable_new',
    inlined from '_unur_mvtdr_triangulate' at ../scipy/_lib/unuran/unuran/src/methods/mvtdr_init.h:978:11:
../scipy/_lib/unuran/unuran/src/methods/mvtdr_init.h:1760:17: warning: argument 1 value '18446744073709551608' exceeds maximum object size 9223372036854775807 [-Walloc-size-larger-than=]
 1760 |   GEN->etable = malloc( size * sizeof(E_TABLE*) );
      |                 ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
In file included from ..\scipy\_lib\unuran\unuran\src/unur_source.h:71,
                 from ../scipy/_lib/unuran/unuran/src/methods/mvtdr.c:50:
../scipy/_lib/unuran/unuran/src/methods/mvtdr_init.h: In function '_unur_mvtdr_triangulate':
c:/rtools40/ucrt64/x86_64-w64-mingw32/include/stdlib.h:531:17: note: in a call to allocation function 'malloc' declared here
  531 |   void *__cdecl malloc(size_t _Size);
      |                 ^~~~~~

[tirthasheshpatel]

This has been changed in unuran 1.9.0. See #2.

GEN->etable is a E_TABLE ** so the change here doesn't seem like the correct fix. The warning thrown by GCC seems to be because of an implicit type conversion issue. Can you please try to build again with changes in #2 and check if the warning persists?