[backtrace] Avoid segfault

[Nathan Sidwell]

I just tripped over a segfault in libbacktrace.  We apply strrchr to a 
possibly NULL filename, with predictable results when it is.

elf.c:3044 passes NULL as the filename parm:
	  ret = elf_add (state, NULL, d, base_address, error_callback, data,
			 fileline_fn, found_sym, found_dwarf, NULL, 0, 1, NULL,
			 0);

This gets to elf_open_debugfile_by_debuglink which passes it on through:
   ddescriptor = elf_find_debugfile_by_debuglink (state, filename,
						 debuglink_name,
						 error_callback, data);

this patch avoids the strrchr when filename is null.  I reordered the 
way prefix & prefix len got set, finding it prefereable to:
   slash  = filename ? NULL : strrchr (filename, '/');
but if you prefer to avoid the assignment in the conditional I'm fine 
with that too.

ok?

nathan

-- 
Nathan Sidwell
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