If the value_line regex matches, and the symbol file happens to exist
as a directory, we'd call addr2line and crash. Fix this so that calling
addr2line on a directory doesn't happen.
There isn't an easy way to test this, but I added a single new unit test. The
real test was to use a tombstone file that used to crash, but no longer crashes.
Test: Wrote new unit test, ran over a tombstone file that used to crash.
Change-Id: I4331e3b032cee8d697d1b7f409638750c61971d6
5f1b4f0d9e