lib-sieve: plugins: enotify: mailto: uri-mailto - Change _is_qchar() to accept char parameter.

It is used like that everywhere and ubsan complains about passing char to
unsigned char parameter.

src/lib-sieve/plugins/enotify/mailto/uri-mailto.c

@@ -134,9 +134,9 @@ static const char _qchar_lookup[256] = {
 	0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,  // F0
 };
 
-static inline bool _is_qchar(unsigned char c)
+static inline bool _is_qchar(char c)
 {
-	return ((_qchar_lookup[c] & 0x01) != 0);
+	return ((_qchar_lookup[(unsigned char)c] & 0x01) != 0);
 }
 
 static inline int _decode_hex_digit(unsigned char digit)