fix for bug #12744 (MYSQL_STMT operations cause seg fault after connection reset)

libmysql/libmysql.c:
  stmt->mysql could be 0x0 if the connection has failed between prepare and execute
  or any other operation. thus if the user decides to use mysql_stmt_reset()
  we should not segfault.
tests/mysql_client_test.c:
  test for bug #12744 (MYSQL_STMT operations cause seg fault after connection reset)

libmysql/libmysql.c

@@ -4628,6 +4628,12 @@ my_bool STDCALL mysql_stmt_reset(MYSQL_STMT *stmt)
   /* If statement hasnt been prepared there is nothing to reset */
   if ((int) stmt->state < (int) MYSQL_STMT_PREPARE_DONE)
     DBUG_RETURN(0);
+  if (!stmt->mysql)
+  {
+    /* mysql can be reset in mysql_close called from mysql_reconnect */
+    set_stmt_error(stmt, CR_SERVER_LOST, unknown_sqlstate);
+    DBUG_RETURN(1);  
+  }
 
   mysql= stmt->mysql->last_used_con;
   int4store(buff, stmt->stmt_id);		/* Send stmt id to server */

tests/mysql_client_test.c

@@ -11706,6 +11706,37 @@ static void test_bug12001()
   DIE_UNLESS(res==1);
 }
 
+static void test_bug12744()
+{
+  MYSQL_STMT *prep_stmt = NULL;
+  int rc;
+  myheader("test_bug12744");
+	
+  prep_stmt= mysql_stmt_init(mysql);
+  rc= mysql_stmt_prepare(prep_stmt, "SELECT 1", 8);
+  DIE_UNLESS(rc==0);
+  
+  rc= mysql_kill(mysql, mysql_thread_id(mysql));
+  DIE_UNLESS(rc==0);
+
+  if (rc= mysql_stmt_execute(prep_stmt))
+  {
+    if (rc= mysql_stmt_reset(prep_stmt))
+      printf("OK!\n");
+    else
+    {
+      printf("Error!");
+      DIE_UNLESS(1==0);      
+    }
+  }
+  else
+  {
+    fprintf(stderr, "expected error but no error occured\n");
+    DIE_UNLESS(1==0);
+  }
+  rc= mysql_stmt_close(prep_stmt);
+}
+
 /*
   Bug#11718: query with function, join and order by returns wrong type
 */
@@ -12054,6 +12085,7 @@ static struct my_tests_st my_tests[]= {
   { "test_bug8378", test_bug8378 },
   { "test_bug9735", test_bug9735 },
   { "test_bug11183", test_bug11183 },
+  { "test_bug12744", test_bug12744 },
   { "test_bug12001", test_bug12001 },
   { "test_bug11718", test_bug11718 },
   { "test_bug12925", test_bug12925 },