[PATCH, Libbacktrace] Fix possible SEGV when handling stripped PIE binaries.

[Maxim Ostapenko]

Hi!

When testing ASan on large system, I've noticed that sometimes it 
crashes with SEGV in Libbacktrace when trying to symbolize stripped PIE 
(compiled with -pie -fPIC) binaries in fully stripped environment (this 
means that all dependent libraries are also stripped). Here a scenario 
I've observed:

1) _asan_backtrace_initialize calls elf_add passing &elf_fileline_fn as 
output parameter to properly initialize it.
2) elf_add doesn't elf_fileline_fn initialize and returns -1 for 
stripped PIE binary.
3) _asan_backtrace_initialize calls phdr_callback on each dependent 
library via dl_iterate_phdr.
4) phdr_callback initializes elf_fileline_fn iff it found debug info in 
some library (found_dwarf == 1), but this is false since all libs are 
stripped. So, we still have uninitialized elf_fileline_fn value.
5) _asan_backtrace_initialize uses elf_fileline_fn to initialize proper 
fileline_fn callback.
6) Libbacktrace uses fileline_fn callback later and crashes because it 
contains garbage.

This patch fixes the issue by simply initializing elf_fileline_fn via 
elf_nodebug in _asan_backtrace_initialize prologue.

Tested on x86_64-linux-gnu and arm-linux-gnueabi, OK for trunk?

-Maxim
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