valgrind: Fix memory leak at expr_error.

Reported by test case 2015: ovn -- action parsing.
    xvasprintf (util.c:164)
    expr_error (expr.c:489)
    expr_parse_field (expr.c:2910)
    action_parse_field (actions.c:287)

Signed-off-by: William Tu <[email protected]>
Signed-off-by: Justin Pettit <[email protected]>

ovn/lib/actions.c

@@ -288,6 +288,7 @@ action_parse_field(struct action_context *ctx,
                              &prereqs);
     if (error) {
         action_error(ctx, "%s", error);
+        free(error);
         return false;
     }