ima: fix buffer overrun in ima_eventdigest_init_common

In the Linux kernel, the following vulnerability has been resolved: ima: fix buffer overrun in ima_eventdigest_init_common Function ima_eventdigest_init() calls ima_eventdigest_init_common() with HASH_ALGO__LAST which is then used to access the array hash_digest_size[] leading to buffer overrun. Have a conditional statement to handle this.