[krbdev.mit.edu #1527] krb5_rd_safe_basic() throws exception when sender_addr is NULL

[Sam Hartman via RT]

Note that s-address is not optional and that krb_mk_safe must fail if
the context does not have address bindings.

So it's only a matter of a segfault vs an error return.  I.E. still a
bug but not a very serious one.