ovsdb: Add default case to ovsdb_mutation_set_execute().

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/).

ovsdb/mutation.c

@@ -379,6 +379,9 @@ ovsdb_mutation_set_execute(struct ovsdb_row *row,
             ovsdb_datum_subtract(dst, dst_type, arg, arg_type);
             error = ovsdb_mutation_check_count(dst, dst_type);
             break;
+
+        default:
+            NOT_REACHED();
         }
         if (error) {
             return error;