Fix potential crashes when Target::make_symbol() returns NULL.

2016-08-10  James Clarke  <jrtc27@jrtc27.com>

gold/
	PR gold/20443
	* symtab.cc (Symbol_table::add_from_relobj): Handle NULL symbols,
	which will be present for STT_SPARC_REGISTER.
	(Symbol_table::add_from_pluginobj): Likewise.
	(Symbol_table::add_from_dynobj): Likewise.
	(Symbol_table::add_from_incrobj): Removed dead code.