Don't crash when no __debug_info section is found

DumpSymbols::ReadDwarf() currently assumes a __debug_info section is always
present and crashes if there isn't one (through using an assert).

Gracefully handle this by returning false if the section isn't present.

[Lei Zhang (chromium)]

Thanks for doing this. If you can sign the CLA per
https://code.google.com/p/google-breakpad/wiki/ContributingToBreakpad, I'd be
happy to commit this for you.