MDEV-23867: insert... select crash in compute_window_func

There are 2 issues here:

Issue #1: memory allocation.
An IO_CACHE that uses encryption uses a larger buffer (it needs space for the encrypted data,
decrypted data, IO_CACHE_CRYPT struct to describe encryption parameters etc).

Issue #2: IO_CACHE::seek_not_done
When IO_CACHE objects are cloned, they still share the file descriptor.
This means, operation on one IO_CACHE may change the file read position
which will confuse other IO_CACHEs using it.

The fix of these issues would be:
Allocate the buffer to also include the extra size needed for encryption.
Perform seek again after one IO_CACHE reads the file.

Author: Varun Gupta

include/my_sys.h

@@ -476,18 +476,19 @@ typedef struct st_io_cache		/* Used when cacheing files */
     partial.
   */
   int	seek_not_done,error;
-  /* buffer_length is memory size allocated for buffer or write_buffer */
+  /* length of the buffer used for storing un-encrypted data */
   size_t	buffer_length;
   /* read_length is the same as buffer_length except when we use async io */
   size_t  read_length;
   myf	myflags;			/* Flags used to my_read/my_write */
   /*
-    alloced_buffer is 1 if the buffer was allocated by init_io_cache() and
-    0 if it was supplied by the user.
+    alloced_buffer is set to the size of the buffer allocated for the IO_CACHE.
+    Includes the overhead(storing key to ecnrypt and decrypt) for encryption.
+    Set to 0 if nothing is allocated.
     Currently READ_NET is the only one that will use a buffer allocated
     somewhere else
   */
-  my_bool alloced_buffer;
+  size_t alloced_buffer;
 #ifdef HAVE_AIOWAIT
   /*
     As inidicated by ifdef, this is for async I/O, which is not currently