Fix decoder handling of floor0 when the LSP order is 1.

Fix decoder handling of floor0 when the LSP order is 1.

Header setup allows the LSP order to be as low as one, but the code in
 vorbis_lsp_to_curve() assumed it was at least two.
This wasn't terrible in libvorbis... it would multiply a nonsense (but defined)
 value into the output, and nothing more.
In Tremor, it referenced several completely undefined (stack) values, which
 could cause out-of-bounds lookup table accesses and crashes.

svn path=/trunk/vorbis/; revision=17538