Prevent "index out of range" crash when addr2line aborts in between

addr2line may abort and "result" may be incomplete. This leads to
"index out of range" exceptions. Enclose this in a try & except block
and on exception, default to function:location = ---:---

Test: "python native_heapdump_viewer_tests.py" (after CL:780974)
      passes.

BUG: 117306194

Change-Id: I4fd5c784949f279550aec95b68045df4a671da74