lib/file: Fix errors found by Coverity scan

lib/file: Fix errors found by Coverity scan

Fix several errors in copy_file_secure_dest() found by Coverity and some
minor formatting issues:

143603: Correctly handle mkstemp() return value
143605: Avoid accessing dest_filename[-1] on readlink() error
143606, 143610: Avoid accessing dest_filename[sizeof(dest_filename)]
143607: Fix incorrectly passing sizeof(pointer) to fread()
143608, 143611: Cleanup resources on early exit
143609: Explicitly set umask before calling mkstemp()

Signed-off-by: Samuel Mendoza-Jonas <sam@mendozajonas.com>