Bug#58765: Warning in item.h on Windows

Truncate the maximum result length (64-bit wide type) to fit into
the item maximum length (32-bit wide type). This is possible as
this specific branch is only used if the maximum result length
is less than 0x1000000 (MAX_BLOB_WIDTH), which fits comfortably
in a 32-bit wide type.

sql/item.h

@@ -1231,7 +1231,7 @@ public:
       maybe_null= 1;
     }
     else
-      max_length= max_result_length;
+      max_length= (uint32) max_result_length;
   }
   void fix_length_and_charset_datetime(uint32 max_char_length_arg)
   {