MFC r279301: GNU nm: Avoid NULL dereference
MFC r279301: GNU nm: Avoid NULL dereference

  bfd_dwarf2_find_line() calls find_line() with NULL functionname_ptr,
  which resulted in a crash on certain ELF objects.

  This change was implemented independently from upstream binutils, but
  I have checked that the crash does not happen there.

Sponsored by:	The FreeBSD Foundation

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