Submitting journal version of an article while conference version is still in review
As a general rule, as long as you are honest with the editor and reviewers, you're OK:


Make it clear in your cover letter that you are submitting an extended version of a paper that is currently under review at a conference. Also clarify this anywhere else in the submission process where you are asked whether this submission is an extended version of an existing publication.
Submit a copy of the conference version together with your journal manuscript, so that reviewers and editors can evaluate the new contribution. Note on this copy that it is currently under review.


Some journals have a policy specifically covering this case. For example, IEEE Transactions on Wireless Communications: 


  It is also possible to submit the journal and conference version at the same time.  Or the journal version can even be submitted first. The journal version should not have appeared, however, at the time of submission of a conference version of that paper.