drivers: wifi: eswifi: Fix coverity issues

This patch fixes the following issues:
CID 190622 (#1 of 1): Out-of-bounds access (OVERRUN)
CID 190632 (#1 of 1): Out-of-bounds access (OVERRUN)
CID 190623 (#1 of 1): Unchecked return value (CHECKED_RETURN)
CID 190628 (#1 of 1): Out-of-bounds write (OVERRUN)
CID 190620 (#1 of 1): Dereference after null check (FORWARD_NULL)

Signed-off-by: Loic Poulain <loic.poulain@linaro.org>