libexif: Avoid accessing invalid data

Exif tags Make and Model can be undefined or
lack string terminators. Check the EXIF size
before trying to compare their values to constant
strings.

Bug: 147140917
Test: run sts-engbuild-no-spl-lock -m StsHostTestCases --test
android.security.sts.Poc20_03#testPocBug_147140917

Change-Id: I255f1937bec99bff7e72482d525f252290d1ddaf