Refactor zic overflow checking

* zic.c (oadd, tadd): Negate sense of branch,
to simplify future changes.
(tadd): Simplify.

zic.c

@@ -3690,28 +3690,19 @@ time_overflow(void)
 static ATTRIBUTE_PURE zic_t
 oadd(zic_t t1, zic_t t2)
 {
-	if (t1 < 0 ? t2 < ZIC_MIN - t1 : ZIC_MAX - t1 < t2)
-	  time_overflow();
-	return t1 + t2;
+  if (t1 < 0 ? ZIC_MIN - t1 <= t2 : t2 <= ZIC_MAX - t1)
+    return t1 + t2;
+  time_overflow();
 }
 
 static ATTRIBUTE_PURE zic_t
 tadd(zic_t t1, zic_t t2)
 {
-  if (t1 < 0) {
-    if (t2 < min_time - t1) {
-      if (t1 != min_time)
-	time_overflow();
-      return min_time;
-    }
-  } else {
-    if (max_time - t1 < t2) {
-      if (t1 != max_time)
-	time_overflow();
-      return max_time;
-    }
-  }
-  return t1 + t2;
+  if (t1 < 0 ? min_time - t1 <= t2 : t2 <= max_time - t1)
+    return t1 + t2;
+  if (t1 == min_time || t1 == max_time)
+    return t1;
+  time_overflow();
 }
 
 /*