addr2line: skip CUs lacking debug info instead of bailing out

Some binaries (such as the FreeBSD kernel) contain a mixture of CUs
with and without debug information. Previously translate() exited upon
encountering a CU without debug information. Instead, just move on to
the next CU.

Reported by: royger
Reviewed by: royger
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D3712