Fix regression in "commands"

Pedro pointed out a regression in "commands", where trying to clear a
breakpoint's command list would fail:

    (top-gdb) commands
    Type commands for breakpoint(s) 3, one per line.
    End with a line saying just "end".
    >end
    No breakpoints specified.
    (top-gdb)

I believe the bug was introduced by my patch that changes
counted_command_line to be a shared_ptr.  This causes the problem
because now the counted_command_line in commands_command_1 can be NULL,
whereas previously it never could be.

After some discussion, we agreed to simply remove the error case from
commands_command_1.

2017-12-07  Tom Tromey  <tom@tromey.com>

	PR breakpoints/22511:
	* breakpoint.c (commands_command_1): Don't throw an exception when
	no commands have been read.

2017-12-07  Tom Tromey  <tom@tromey.com>

	* gdb.base/break.exp: Add test for empty "commands".

gdb/breakpoint.c

@@ -1272,9 +1272,6 @@ commands_command_1 (const char *arg, int from_tty,
 	   observer_notify_breakpoint_modified (b);
 	 }
      });
-
-  if (cmd == NULL)
-    error (_("No breakpoints specified."));
 }
 
 static void