checkpatch: fix premature exit when no input or --mailback

In some cases, checkpatch's process subroutine is exiting the
whole process.  This is wrong, just return from the subroutine
instead.

Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>