ofproto: Avoid wasting memory malloc()'ing empty action sets for subrules.

ofproto: Avoid wasting memory malloc()'ing empty action sets for subrules.

GNU libc treats malloc(0) as malloc(1).  Subrules always have an n_actions
of 0, so this code was wasting time and memory for subrules.  This commit
stops doing that.

Also audits and fixes some very pedantic potential problems with null
pointers; e.g. the C standard says that NULL may not be compared with the
< operator, even if both arguments are null, and it also says that a null
pointer may not be passed to memcpy() or memcmp(), even if the length is
zero.