wmail: fixed NULL-pointer dereference if opendir(3) fails.

Code was:

  if (( dir = opendir( ... )))
  {
    // ...
  }
  closedir( dir ); // dir may be NULL

Moved closedir(3) inside if-block.

wmail/src/wmail.c

@@ -981,9 +981,9 @@ static int TraverseDirectory( const char *name, bool isNewMail )
 	    }
 	    free( fullName );
 	}
-    }
 
-    closedir( dir );
+	closedir( dir );
+    }
 
     return mails;
 }