protocol/ArgParser: check for invalid ranges

Catch errors like that early, before invalid ranges get passed to
internal MPD subsystems.

NEWS

@@ -1,4 +1,6 @@
 ver 0.22.5 (not yet released)
+* protocol
+  - error for malformed ranges instead of ignoring silently
 * database
   - simple: fix missing CUE sheet metadata in "addid" command
 * tags

src/protocol/ArgParser.cxx

@@ -110,6 +110,10 @@ ParseCommandArgRange(const char *s)
 		return RangeArg::Single(range.start);
 	}
 
+	if (!range.IsWellFormed())
+		throw FormatProtocolError(ACK_ERROR_ARG,
+					  "Malformed range: %s", s);
+
 	return range;
 }