Fix signed overflow check in k5_ucs2s_to_utf8s

Signed overflow must be checked before it happens, since modern
versions of gcc will optimize out checks of the result.  Reported by
Nickolai Zeldovich <nickolai@csail.mit.edu>.

https://github.com/krb5/krb5/commit/7506becc0ac70915050e097d673e7647b99347fc
Author: Greg Hudson <ghudson@mit.edu>
Commit: 7506becc0ac70915050e097d673e7647b99347fc
Branch: master
 src/util/support/utf8_conv.c |    4 +---
 1 files changed, 1 insertions(+), 3 deletions(-)