Bug#23010 _my_b_read() passing illegal file handles to my_seek()

- The io cache flag seek_not_done was not set properly in the 
  reinit_io_cache function call and this led my_seek to be called
  desipite an invalid file handle.
- Added a test in reinit_io_cache to ensure we have a valid file 
  handle before setting seek_not_done flag.

mysys/mf_iocache.c

@@ -313,7 +313,11 @@ my_bool reinit_io_cache(IO_CACHE *info, enum cache_type type,
       if (info->type == READ_CACHE)
       {
 	info->write_end=info->write_buffer+info->buffer_length;
-	info->seek_not_done=1;
+        /*
+          Trigger a new seek only if we have a valid
+          file handle.
+        */
+        info->seek_not_done= (info->file >= 0);
       }
       info->end_of_file = ~(my_off_t) 0;
     }

mysys/my_seek.c

@@ -30,6 +30,11 @@ my_off_t my_seek(File fd, my_off_t pos, int whence,
 		   whence, MyFlags));
   DBUG_ASSERT(pos != MY_FILEPOS_ERROR);		/* safety check */
 
+  /*
+    Make sure we are using a valid file descriptor
+  */
+  DBUG_ASSERT(fd >= 0);
+  
   newpos=lseek(fd, pos, whence);
   if (newpos == (os_off_t) -1)
   {