glDrawBuffers(n==0) is valid

glDrawBuffers(n==0) is valid

According to the GL spec, calling glDrawBuffers() with n == 0 is a
valid operation (and essentially prevents drawing to any buffers).
But _msa_DrawBuffersARB() was producing a GL_INVALID_VALUE error in
this case.

This fix adjusts the error check, and makes a small change to the
ctx->Driver.DrawBuffer() call below to ensure that, if n == 0,
Driver.DrawBuffer() is called with GL_NONE and that buffers[0] is
*not* referenced in this case (since we don't know whether it is valid).

Internal identifier: 365833