check to make sure "trades" data exists.  In pre-market, this data is sometimes missing; now you get NA instead of an ambiguous error.

git-svn-id: svn://scm.r-forge.r-project.org/svnroot/twsinstrument/pkg/qmao@393 edb9625f-4e0d-4859-8d74-9fd3b1da38cb