Be liberal about what to accept

This addressee a bug where ardour 6 was able to write negative
duration `length="-1"` `length-beats="-3.3650500597559585e-05"`

Ideally timecnt_t::string_to should check for invalid,
negative, duration. But this also catches a more generic case.

```
exception at str.substr (1)
#3 Temporal::timepos_t::string_to (this=0x7fffffff7bb0, str="") at libs/temporal/timeline.cc:904
#4 Temporal::timecnt_t::string_to (this=0x7fffffff7ba0, str="-2") at libs/temporal/timeline.cc:294
#5 PBD::string_to<Temporal::timecnt_t> (str="-2") at libs/ardour/ardour/types_convert.h:131
```

libs/temporal/timeline.cc

@@ -280,6 +280,8 @@ timecnt_t::string_to (std::string const & str)
 		ss >> ticks;
 		_distance = int62_t (true, ticks);
 		break;
+	default:
+		return false;
 	}
 
 	/* eat separator character */