The control flow here is going to become more complicated, and this
change will make the next patches, and the resulting code, clearer.
Note that process_msg0's return value is never used; it is only
defined to return bool_t so that it can use the CHECK_AVAIL macro.
Knowing this will make it easier to see that the patch is correct.
Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>