[krbdev.mit.edu #3917] krb5-1.5: fix warnings in lib/crypto/arcfour/ and lib/crypto/yarrow/

[Public Submitter via RT]

warning fixes for:
arcfour_s2k.c:46: warning: pointer targets in passing argument 2 of 
'asctouni' differ in signedness

ycipher.c:77: warning: pointer targets in assignment differ in 
signedness