usr.bin/diff: fix UBSan error in readhash

UBSan complains about the `sum = sum * 127 + chrtran(t);` line below since
that can overflow an `int`. Use `unsigned int` instead to ensure that
overflow is well-defined.

Reviewed By:	imp
MFC after:	1 week
Differential Revision: https://reviews.freebsd.org/D31075

(cherry picked from commit c78f449d85)

usr.bin/diff/diffreg.c

@@ -1345,7 +1345,7 @@ static int
 readhash(FILE *f, int flags)
 {
 	int i, t, space;
-	int sum;
+	unsigned sum;
 
 	sum = 1;
 	space = 0;