Avoid double-free with debuginfod

Avoid double-free with debuginfod

PR gdb/29257 points out a possible double free when debuginfod is in
use.  Aside from some ugly warts in the symbol code (an ongoing
issue), the underlying issue in this particular case is that elfread.c
seems to assume that symfile_bfd_open will return NULL on error,
whereas in reality it throws an exception.  As this code isn't
prepared for an exception, bad things result.

This patch fixes the problem by introducing a non-throwing variant of
symfile_bfd_open and using it in the affected places.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=29257