Fix clang 10 warnings

_ma_fetch_keypage(): Correct an assertion that used to always hold.
Thanks to clang -Wint-in-bool-context for flagging this.

double_to_datetime_with_warn(): Suppress -Wimplicit-int-float-conversion
by adding a cast. LONGLONG_MAX converted to double will actually be
LONGLONG_MAX+1.

sql/sql_time.cc

 /* Copyright (c) 2000, 2010, Oracle and/or its affiliates.
-   Copyright (c) 2009, 2013 Monty Program Ab.
+   Copyright (c) 2009, 2020, MariaDB Corporation.
 
    This program is free software; you can redistribute it and/or modify
    it under the terms of the GNU General Public License as published by
@@ -402,7 +402,7 @@ bool double_to_datetime_with_warn(double value, MYSQL_TIME *ltime,
   if (neg)
     value= -value;
 
-  if (value > LONGLONG_MAX)
+  if (value > static_cast<double>(LONGLONG_MAX))
     value= static_cast<double>(LONGLONG_MAX);
 
   longlong nr= static_cast<ulonglong>(floor(value));

storage/maria/ma_page.c

 /* Copyright (C) 2006 MySQL AB & MySQL Finland AB & TCX DataKonsult AB
+   Copyright (c) 2020, MariaDB Corporation.
 
    This program is free software; you can redistribute it and/or modify
    it under the terms of the GNU General Public License as published by
@@ -112,7 +113,7 @@ my_bool _ma_fetch_keypage(MARIA_PAGE *page, MARIA_HA *info,
 
   if (lock != PAGECACHE_LOCK_LEFT_UNLOCKED)
   {
-    DBUG_ASSERT(lock == PAGECACHE_LOCK_WRITE || PAGECACHE_LOCK_READ);
+    DBUG_ASSERT(lock == PAGECACHE_LOCK_WRITE || lock == PAGECACHE_LOCK_READ);
     page_link.unlock= (lock == PAGECACHE_LOCK_WRITE ?
                        PAGECACHE_LOCK_WRITE_UNLOCK :
                        PAGECACHE_LOCK_READ_UNLOCK);