target/xtensa: fix final clang analyzer warning

Reworked xtensa_read_memory() logic to always allocate
and initialize working buffer with sufficient padding.

Signed-off-by: Ian Thompson <ianst@cadence.com>
Change-Id: Ia9ab53336537adebf99f8156f481ca8279a7cd5d
Reviewed-on: https://review.openocd.org/c/openocd/+/7211
Tested-by: jenkins
Reviewed-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-by: Erhan Kurubas <erhan.kurubas@espressif.com>