gdb segv in elfread.c:elf_rel_plt_read

After commit 68bbe1183379, ELF symbols read via bfd_canonicalize_symtab
and similar functions which have bad st_name fields will have NULL in
the name rather than "(null)".  gdb.base/bfd-errors.exp deliberately
creates a faulty shared library with st_name pointing outside of
.dynsym for some symbols, and thus now results in NULL symbol names.
This triggers a segv on string_buffer.assign(name).  Fix that.