fixed gfx_AllocSprite return value bug #659
                
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MERGE #657 BEFORE THIS!!!
fixes #658
gfx_AllocSprite made an invalid assumption that the allocation routine would preserve the stack parameters. allocation routines such as
__simple_mallocdestroy the stack parameter, which can cause a returned non-NULL pointer to be off by one. This is especially problematic when this off-by-one pointer has to be freed.The bug with the
width * height == 0case is assumed to be undefined behavior, since the sprite routines don't handle this case either.