Avoid crashing recovery with unwritable /cache.

When /cache is unwritable, recovery hits a crash loop. Because it
passes nullptr to fileno(3) when writing back the locale file. This
prevents user from recovering a device - it cannot boot far enough to
recovery menu which allows wiping /cache.

Bug: 63927337
Test: Corrupt /cache and boot into recovery on bullhead:
 1. m -j recoveryimage
 2. fastboot erase cache
 3. fastboot boot $OUT/recovery.img
 4. recovery menu shows up.
Change-Id: I1407743f802049eb48add56a36298b665cb86139
(cherry picked from commit ec57903a7ec0bfe3c2f39dd6ee9cfc3de4ed20e6)