Fix for BUG#7714 "if disk full, sometimes MyISAM doesn't wait for free space, corrupts table"

This happened only if my_write() couldn't write even one byte.
I cannot easily add a .test for this, but I tested by hand before and after the change.

mysys/my_write.c

@@ -49,8 +49,7 @@ uint my_write(int Filedes, const byte *Buffer, uint Count, myf MyFlags)
       MyFlags&= ~ MY_WAIT_IF_FULL;		/* End if aborted by user */
 #endif
     if ((my_errno == ENOSPC || my_errno == EDQUOT) &&
-        (MyFlags & MY_WAIT_IF_FULL) &&
-	(uint) writenbytes != (uint) -1)
+        (MyFlags & MY_WAIT_IF_FULL))
     {
       if (!(errors++ % MY_WAIT_GIVE_USER_A_MESSAGE))
 	my_error(EE_DISK_FULL,MYF(ME_BELL | ME_NOREFRESH),