main: Fix target checking for CopyTexSubImage*D.
This fixes a dEQP test failure. In the test,
glCopyTexSubImage2D was called with target = 0 and failed to throw
INVALID ENUM. This failure was caused by _mesa_get_current_tex_object(ctx,
target) being called before the target checking. To remedy this, target
checking was separated from the main error-checking function and
called prior to _mesa_get_current_tex_object.
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=89312
Reviewed-by: Anuj Phogat <anuj.phogat@gmail.com>
(cherry picked from commit
ca65764d6042d2ea220a1e3952490f79c226f3e0)