When updating the install list for files which have had local changes

merged with upgrade changes, don't try to compute the SHA256 hash of
files which don't exist.

Reported by:	Jaakko Heinonen
MFC after:	1 week

usr.sbin/freebsd-update/freebsd-update.sh

@@ -2196,9 +2196,9 @@ EOF
 
 		# Store merged files.
 		while read F; do
-			V=`${SHA256} -q merge/new/${F}`
-
 			if [ -f merge/new/${F} ]; then
+				V=`${SHA256} -q merge/new/${F}`
+
 				gzip -c < merge/new/${F} > files/${V}.gz
 				echo "${F}|${V}"
 			fi