Check an edge case when read(2) returns 0

We might end up in an infinite loop if read(2) reached EOF unexpectedly.
The problematic code in uncrypt mentioned in the bug has been fixed
by switching to libbase ReadFully(). So I grepped through the recovery
code and fixed some other occurences of the issue.

Bug: 31073201
Change-Id: Ib867029158ba23363b8f85d61c25058a635c5a6b