Fix debug assert to match its intention.

Do not check my_errno before it is set, check errno instead.
Also, do not check errno, if prior pread() did not fail.

mysys/my_pread.c

@@ -69,7 +69,7 @@ size_t my_pread(File Filedes, uchar *Buffer, size_t Count, my_off_t offset,
     if (readbytes != Count)
     {
       /* We should never read with wrong file descriptor! */
-      DBUG_ASSERT(my_errno != EBADF);
+      DBUG_ASSERT(readbytes != (size_t)-1 || errno != EBADF);
       my_errno= errno;
       if (errno == 0 || (readbytes != (size_t) -1 &&
                          (MyFlags & (MY_NABP | MY_FNABP))))