mesa/main: Check for 0 size draws after validation.

mesa/main: Check for 0 size draws after validation.

When validating draw parameters move check for 0 draw count last
(drawing with count 0 is not an error), so that other parameters (e.g.: the
primitive type) are validated and the correct errors (if applicable) are
generated.

>From the OpenGL 3.3 spec page 33 (page 48 of the PDF):
"[Regarding DrawArraysOneInstance, in terms of which other draw operations
are defined:]
If count is negative, an INVALID_VALUE error is generated."

This patch also changes the bahavior of MultiDrawElements to perform the draw
operation if some primitive's index counts are zero.

Signed-off-by: Fabian Bieler <fabianbieler@fastmail.fm>
Reviewed-by: Brian Paul <brianp@vmware.com>
Reviewed-by: Paul Berry <stereotype441@gmail.com>