secchan: Correctly maintain rule's number of actions.

secchan: Correctly maintain rule's number of actions.

We were reallocating memory properly but not updating the "n_actions"
member, which sometimes led to reading past the end of a block.

This problem possibly caused segfaults, since the OpenFlow->ODP action
translation code assumes that OpenFlow actions are prevalidated, and
random memory is not necessarily valid OpenFlow actions :-)

Spotted by Valgrind.