main: Fix target checking for CopyTexSubImage*D.

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>