The value of 'error' is indeterminate if m->mutator is not set to a valid
value here, so we should add a default case to handle the exception. It
shouldn't happen, of course.
Found by Clang (http://clang-analyzer.llvm.org/).
This commit refactors the functions for working with "struct ovsdb_datum",
adding and exposing some more operations for ovs-vsctl to use in an
upcoming commit.