crypto: testmgr - change `guard` to unsigned char

When char is signed, storing the values 0xba (186) and 0xad (173) in the
`guard` array produces signed overflow. Change the type of `guard` to
static unsigned char to correct undefined behavior and reduce function
stack usage.

Signed-off-by: Joey Pabalinas <joeypabalinas@gmail.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>