Core: Fix OOB

getExternal___Dir() relies on getExternal___Dirs to return an array
of at least 1 element.
Make sure there is at least one element returned.

(Reference:
BugDump 13-20160513-19 l#45 (FileManager) and L#104 (Gallery)
BugDump 13-20160520-26 L#18 and more (26), just search for
"getExternal" to find all cases of that OOB (>>100 users))

Change-Id: I56394db135c4c53c972e3bcc8f1df4ea1d4d39e7