tweaks: use a more effecient way to skip storing an empty file name

Position history refers only to actual files, so when the filename
is blank...  there is nothing that needs to be remembered.

src/history.c

@@ -528,7 +528,7 @@ void update_poshistory(char *filename, ssize_t lineno, ssize_t xpos)
 	poshiststruct *posptr, *theone, *posprev = NULL;
 	char *fullpath = get_full_path(filename);
 
-	if (fullpath == NULL || fullpath[strlen(fullpath) - 1] == '/') {
+	if (fullpath == NULL || *filename == '\0') {
 		free(fullpath);
 		return;
 	}