Call realpath(3) only when the path is accessible for read

Suppress the SELinux denial log spam by not calling realpath(3) when the
path does not exist or is not accessible for read, and then not auditing
access(2) failure.

Bug: 120996057
Test: copy ping to /data/local/tmp, run it, verify no errors
Test: run bionic-unit-tests, the tests pass
Change-Id: Ie6058bfc9524a9b5c50fc7183fdddea6a8fb9200