appl/afsutil: expand_cell_name return NULL if not found

In function ‘afslog_cell.isra’:
  afslog.c:144:13: warning: ‘%s’ directive argument is null [-Wformat-overflow=]
  144 |            warnx("No cell matching \"%s\" found.", cell);
      |             ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

caused by expand_cell_name() returning the input pointer.
Alter the behavior of expand_cell_name() to match its usage.

Change-Id: I3b15c4b3e59b597af5351e5e62f5d7751be50feb

appl/afsutil/afslog.c

@@ -118,7 +118,7 @@ expand_cell_name(const char *cell)
 	if(c)
 	    return c;
     }
-    return cell;
+    return NULL;
 }
 
 static void