- buck2obj_alloc() initially allocates no buffers

- buck2obj_alloc() initially allocates no buffers
- buck2obj_realloc() only stretches the main buffer (and not the lizard)
  + it is not exported anymore
- if lizard_decompress_safe() returns EFBIG, its buffer is shrinked

hence buck2obj.c allocates no memory if not needed.  the internal buffer is
allocated when the first non-compact bucket is encountered and lizard's
buffer is allocated when the first compressed bucket is encountered.