HKDF infinite loop bug

[david-koenig]

https://github.com/awslabs/aws-encryption-sdk-c/blob/bf26105520772b1b6ac486d978c1325bd13f5a6f/source/hkdf.c#L79-L81

If n has the maximum value of 255, the comparison idx <= n is always true, since idx is a uint8_t, so the loop will never terminate. One way to fix is to make idx an int and assign its value to a uint8_t inside the loop.

Fix should include a test that it works properly when the full 255 hash blocks are needed.