Remove strerror() calls from k5_get_error()

Remove strerror() calls from k5_get_error()

Coverity models strerror() as a function which cannot accept negative
values, even though it has defined behavior on all integers.
k5_get_error() contains code to call strerror_r() and strerror() if
its fptr global is unset, which isn't an expected case in practice.
To silence a large number of Coverity false positives, just return a
fixed string if fptr is null.