Fix uninitialized variable in trustzone secure

The block_idx_end variable is left uninitialized and contains an
undeterminate value. This can cause issues later when it is compared to
block_idx because the comparison may not have the desired outcome.

To resolve this, the block_idx_end variable is now zero initialized.

Change-Id: I0767eeb4dc18ea668e6f14a386b67e4ddb9c654a
Signed-off-by: Mikael Olsson <mikael.olsson@arm.com>