fix mpd crash on invalid utf8 stream title

NEWS

 ver 0.19.12 (not yet released)
+* fix assertion failure on malformed UTF-8 tag
 * fix build failure on non-Linux systems
 * fix LimitRTTIME in systemd unit file
 

src/tag/TagString.cxx

@@ -40,9 +40,9 @@ FindInvalidUTF8(const char *p, const char *const end)
 		/* now call the other SequenceLengthUTF8() overload
 		   which also validates the continuations */
 		const size_t t = SequenceLengthUTF8(p);
-		assert(s == t);
 		if (t == 0)
 			return p;
+		assert(s == t);
 
 		p += s;
 	}