[cmdline] Do not ignore empty initial arguments in concat_args()

Reported-by: Oliver Rath <rath@mglug.de>
Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/core/exec.c

 	ptr = string;
 	for ( arg = args ; *arg ; arg++ ) {
 		ptr += sprintf ( ptr, "%s%s",
-				 ( ( ptr == string ) ? "" : " " ), *arg );
+				 ( ( arg == args ) ? "" : " " ), *arg );
 	}
 	assert ( ptr < ( string + len ) );