Fix princflags memory management

Fix princflags memory management

Fix some out of memory error cases (found by Coverity) that could
cause multiple frees or freeing of invalid pointers.  In
krb5_flagnum_to_string(), don't assume that asprintf() stores a null
pointer on failure (it does in BSD but not in glibc).  In
krb5_flags_to_strings(), free the correct pointer in the cleanup loop
in on error.

ticket: 8215