[ELF] Prevent crash when reporting errors if debug line cannot be parsed

LLD uses the debug info and debug line sections to determine the location of
e.g. references to undefined symbols, when producing error messages. In the
event that debug info was present, but debug line parsing failed for some
reason, then a nullptr would end up being dereferenced by the location-lookup
code.

Differential Revision: https://reviews.llvm.org/D44205

Reviewers: grimar
llvm-svn: 326899