wbemprox: Fix a string leak (Valgrind).

Signed-off-by: Sven Baars <sven.wine@gmail.com>
Signed-off-by: Hans Leidekker <hans@codeweavers.com>
Signed-off-by: Alexandre Julliard <julliard@winehq.org>

dlls/wbemprox/query.c

@@ -632,6 +632,7 @@ done:
     if (query) release_query( query );
     free_path( path );
     SysFreeString( antecedent );
+    heap_free( str );
     return hr;
 }