draw: (trivial) fix out-of-bounds vector initialization

draw: (trivial) fix out-of-bounds vector initialization

Was off-by-one. llvm says inserting an element with an index higher than the
number of elements yields undefined results. Previously such inserts were
ignored but as of llvm revision 235854 the vector gets replaced with undef,
causing failures.
This fixes piglit gl-3.2-layered-rendering-gl-layer, as mentioned in
https://llvm.org/bugs/show_bug.cgi?id=23424.

Reviewed-by: Brian Paul <brianp@vmware.com>
Cc: mesa-stable@lists.freedesktop.org