Fix rematch()

Rematch() was incremnenting the last match always, even in the
cases when it was not matching anything. This patch detects
this situation and it updates it only when there is a match.

ed.c

@@ -404,8 +404,13 @@ match(int num)
 static int
 rematch(int num)
 {
-	lastmatch += matchs[0].rm_eo;
-	return !regexec(pattern, lastmatch, 10, matchs, 0);
+	regoff_t off = matchs[0].rm_eo;
+
+	if (!regexec(pattern, lastmatch + off, 10, matchs, 0)) {
+		lastmatch += off;
+		return 1;
+	}
+	return 0;
 }
 
 static int