Fix internal error from command line with unbalanced --start-lib/--end-lib.

Fix internal error from command line with unbalanced --start-lib/--end-lib.

The problem is that while the command line isn't trivially empty,
it contains no input files. As gold tries to configure the number
of threads to use based on the number of input files, this causes
the assertion failure above. Fix this problem by making the logic
in gold.cc more robust and also adding a better error message
about --start-lib to options.cc.

gold/
	PR gold/22406
	* gold.cc (queue_initial_tasks) Check for number of real input files.
	* options.cc (Command_line::process) Check for unterminated --start-lib
	options.
	* testsuite/Makefile.am: Add new test script.
	* testsuite/Makefile.in: Regenerate.
	* testsuite/check_empty_command_lines.sh: New test script.