$ cat signed-overflow.patch

--- a/roff.c	2021-09-23 19:03:23.000000000 +0100
+++ b/roff.c	2024-05-15 19:25:45.375186003 +0100
@@ -2480,7 +2480,8 @@
 			p++;
 
 	for (*res = 0; isdigit((unsigned char)v[p]); p++)
-		*res = 10 * *res + v[p] - '0';
+		*res = (int) ((unsigned int) 10 * *res + v[p] - '0');
+
 	if (p == *pos + n)
 		return 0;
 
@@ -3062,7 +3063,7 @@
 	}
 
 	if ('+' == sign)
-		reg->val += val;
+		reg->val = (int) ((unsigned int) val + reg->val);
 	else if ('-' == sign)
 		reg->val -= val;
 	else