tweaks: stop checking for a NULL result from line_from_number()

No one ever reported seeing the message that was removed in commit
c039aaad one month ago, so most likely the code is correct and nano
never calls line_from_number() with a non-existent line number.

src/text.c

@@ -523,11 +523,8 @@ void do_undo(void)
 		return;
 	}
 
-	if (u->type <= REPLACE) {
+	if (u->type <= REPLACE)
 		f = line_from_number(u->mark_begin_lineno);
-		if (f == NULL)
-			return;
-	}
 
 	openfile->current_x = u->begin;
 	switch (u->type) {
@@ -695,11 +692,8 @@ void do_redo(void)
 	while (u->next != openfile->current_undo)
 		u = u->next;
 
-	if (u->type <= REPLACE) {
+	if (u->type <= REPLACE)
 		f = line_from_number(u->mark_begin_lineno);
-		if (f == NULL)
-			return;
-	}
 
 	switch (u->type) {
 	case ADD:

src/utils.c

@@ -507,10 +507,7 @@ linestruct *line_from_number(ssize_t lineno)
 		while (f->lineno != lineno && f->next != NULL)
 			f = f->next;
 
-	if (f->lineno == lineno)
-		return f;
-	else
-		return NULL;
+	return f;
 }
 #endif /* !NANO_TINY */