mesa: fix check for dummy renderbuffer in _mesa_FramebufferRenderbufferEXT()

mesa: fix check for dummy renderbuffer in _mesa_FramebufferRenderbufferEXT()

According to the spec:
	<renderbuffertarget> must be RENDERBUFFER and <renderbuffer>
	should be set to the name of the renderbuffer object to be
	attached to the framebuffer.  <renderbuffer> must be either
	zero or the name of an existing renderbuffer object of type
	<renderbuffertarget>, otherwise an INVALID_OPERATION error is
	generated.

This patch changes the previous returned GL_INVALID_VALUE to
GL_INVALID_OPERATION.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=76894

Cc: mesa-stable@lists.freedesktop.org
Reviewed-by: Anuj Phogat <anuj.phogat@gmail.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Signed-off-by: Samuel Iglesias Gonsalvez <siglesias@igalia.com>