Passing uninitialized pointer to the buffer allocator is
not problematic theoretically (as far as the returned
pointer is checked before writing), but g++4.6 dislikes
it and warns by -Wuninitialized.  Initialize them by NULL.

* src/base/ftobjs.c (FT_Stream_New): Init `stream'.
(new_memory_stream): Ditto.
(FT_New_GlyphSlot): Init `slot'.
(FT_CMap_New): Init `cmap'.
(open_face_PS_from_sfnt_stream): Init `sfnt_ps'.
(Mac_Read_POST_Resource): Init `pfb_data'.
(Mac_Read_sfnt_Resource): Init `sfnt_data'.
* src/base/ftrfork.c (FT_Raccess_Get_DataOffsets):
Init `offsets_internal' and `ref'.
(raccess_guess_darwin_hfsplus): Init `newpath'.
(raccess_guess_darwin_newvfs): Ditto.
* src/base/ftbitmap.c (ft_bitmap_assure_buffer):
Init `buffer'.
* src/base/ftstroke.c (FT_Stroker_New): Init `stroker'.