Casting of uint32 to int Will Overflow In 32-bit OS (GSR-18)

[danforbes]

The function verifyPreRuntimeDigest() which verifies a digest message received from the network. It ensure the authority ID received in the message is in the current authority list by checking the ID is less than the length.

An issue arises on 32-bit operating systems where int is a 32 bit signed integer. Casting a uint32 which is larger than 2^31 will result in a negative int . A negative number will not trigger the conditional statement in verifyPreRuntimeDigest(). The function will continue to execute and later use authIdx as an index to the b.authorities list which will panic due to an Index Out Of Bounds error.

Consider modifying the conditional statement in verifyPreRuntimeDigest() to include negative numbers.