added "length" parameter to validUtf8String()

At several places, we create temporary copies of non-null-terminated
strings, just to use them in functions like validUtf8String().  We can
save this temporary allocation and avoid heap fragmentation if we
add a length parameter instead of expecting a null-terminated string.

src/path.c

@@ -47,7 +47,7 @@ static char *path_conv_charset(char *dest, const char *to,
 char *fs_charset_to_utf8(char *dst, const char *str)
 {
 	char *ret = path_conv_charset(dst, "UTF-8", fsCharset, str);
-	return (ret && !validUtf8String(ret)) ? NULL : ret;
+	return (ret && !validUtf8String(ret, strlen(ret))) ? NULL : ret;
 }
 
 char *utf8_to_fs_charset(char *dst, const char *str)