No point in checking if the size of the subbuffer > 0, the code handles it correclty as demonstrated by unit test.

Original commit message from CVS:
* gst/gstbuffer.c: (_gst_buffer_copy), (gst_buffer_create_sub):
* tests/check/gst/gstbuffer.c: (GST_START_TEST),
(gst_buffer_suite):
No point in checking if the size of the subbuffer > 0, the
code handles it correclty as demonstrated by unit test.
Also add a unit test for the zero sized _new_and_alloc and
_copy. Fixes #346663.