Backporting fix from trunk (revid 3381), original comment:

Blind attempt to fix BUG 12881278 - MAIN.MYISAM TEST FAILS ON LINUX
  
  The printed text is truncated on char 63:
  
    "MySQL thread id 1236, OS thread handle 0x7ff187b96700, query id"
  
  still I do not understand how this truncation could have caused the
  main.myisam failure but anyway - the buffer needs to be increased.

sql/sql_class.cc

@@ -641,7 +641,7 @@ char *thd_security_context(THD *thd, char *buffer, unsigned int length,
 {
   String str(buffer, length, &my_charset_latin1);
   const Security_context *sctx= &thd->main_security_ctx;
-  char header[64];
+  char header[256];
   int len;
   /*
     The pointers thd->query and thd->proc_info might change since they are