pref: do not allow -o .v

vlib/v/pref/pref.v

@@ -244,6 +244,10 @@ pub fn parse_args(args []string) (&Preferences, string) {
 			}
 			'-o' {
 				res.out_name = cmdline.option(current_args, '-o', '')
+				if res.out_name.ends_with('.v') {
+					eprintln('Cannot save output binary in a .v file.')
+					exit(1)
+				}
 				i++
 			}
 			'-b' {