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_chache function call and this led my_seek to be called despite 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

@@ -333,7 +333,11 @@ my_bool reinit_io_cache(IO_CACHE *info, enum cache_type type,
     {
       info->read_end=info->write_pos;
       info->end_of_file=my_b_tell(info);
-      info->seek_not_done=1;
+      /*
+        Trigger a new seek only if we have a valid
+        file handle.
+      */
+      info->seek_not_done= (info->file != -1);
     }
     else if (type == WRITE_CACHE)
     {

mysys/my_seek.c

@@ -29,8 +29,13 @@ my_off_t my_seek(File fd, my_off_t pos, int whence,
 		   whence, MyFlags));
   DBUG_ASSERT(pos != MY_FILEPOS_ERROR);		/* safety check */
 
-  if (-1 != fd)
-    newpos=lseek(fd, pos, whence);
+  /*
+      Make sure we are using a valid file descriptor!
+  */
+  DBUG_ASSERT(fd != -1);
+
+  newpos= lseek(fd, pos, whence);
+  
   if (newpos == (os_off_t) -1)
   {
     my_errno=errno;