sparc64 segfault in finish_dynamic_symbol

SYMBOL_REFERENCES_LOCAL can return true for undefined symbols.  This
can result in a segfault when running sparc64 ld/testsuite/ld-vsb
tests that expect a failure.

	* elfxx-sparc.c (_bfd_sparc_elf_finish_dynamic_symbol): Don't
	access u.def.section on non-default visibility undefined symbol.