MDEV-27297 wsrep error log messages drop last character

vsnprintf takes the space need for trailing '\0' in consideration, and copies only n-1 characters to destination buffer.
With the old code, only sizeof(buf)-2 characters were copied, this caused that last character of message could be lost.
Reviewed-by: Jan Lindström <jan.lindstrom@mariadb.com>

sql/wsrep_mysqld.cc

@@ -288,7 +288,7 @@ void WSREP_LOG(void (*fun)(const char* fmt, ...), const char* fmt, ...)
   char msg[128] = {'\0'};
   va_list arglist;
   va_start(arglist, fmt);
-  int n= vsnprintf(msg, sizeof(msg) - 1, fmt, arglist);
+  int n= vsnprintf(msg, sizeof(msg), fmt, arglist);
   va_end(arglist);
   if (n < 0)
   {