history: save the positions file also when a record was deleted

When a file is closed with the cursor on line 1, column 1, this
position is not recorded in file-positions history file -- if a
record for the file existed, it is deleted.  In the latter case
the history file needs to be saved, so that other instances of
nano will know about the deletion.

This fixes https://savannah.gnu.org/bugs/?52505.

src/history.c

@@ -548,6 +548,7 @@ void update_poshistory(char *filename, ssize_t lineno, ssize_t xpos)
 		posprev->next = posptr->next;
 	    free(posptr->filename);
 	    free(posptr);
+	    save_poshistory();
 	}
 	free(fullpath);
 	return;