owl_message_create_from_znotice might crash if z_opcode is NULL?

[andersk@mit.edu]

clang scan-build finds that owl_message_create_from_znotice passes n->z_opcode to strcasecmp even though it previously tested it for being NULL. If that ever happens, it might lead to a crash in strcasecmp. If that never happens, we should remove the NULL test.