util/draw: replace assertions with conditionals in util_draw_max_index()

util/draw: replace assertions with conditionals in util_draw_max_index()

Don't assert/die if a VBO is too small.  Return zero instead.  For
debug builds, emit a warning message since this is an unusual situation
that might indicate that there's a bug in the app.

Note that util_draw_max_index() now returns max_index+1 instead of
max_index.  This lets us return zero to indicate that one of the VBOs
is too small to draw anything.

Fixes a failure with the new piglit vbo-too-small test.

Reviewed-by: José Fonseca <jfonseca@vmware.com>