Fixes an issue with no database selected
Fixes an issue with no database selected

Summary:
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Not exactly sure how to reproduce the issue. Having said that:
- A database id is a non-zero number (1..n)
- If there's no selected database and/or there's no databases, then selectedDatabase is '0', which is an invalid database id.
- It is safer to check if the selected database id falls within bounds before attempting to obtain the tables from it.

From Logview, if the database id is '0', which is invalid, then we attempt to access a database at index -1 (database[selectedDatabase - 1]) which is definitely invalid. The returned object is undefined and hence the error.

Changelog: Fixes an issue on the databases plugin when there is no selected database.

Reviewed By: mweststrate

Differential Revision: D35810827

fbshipit-source-id: 4c9f112eebcd0aa3fcd5df316749f48f3922381c

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