filters: fix incorrect filter indexing in composite filter array

filters: fix incorrect filter indexing in composite filter array

Caused by recent changes - a7be3b12dfe7388d1648595e6cc4c7a1379bb8a7.
If global filter was not defined, then part of the code
creating composite filter (the cmd->lvmetad_filter) incorrectly
increased index value even if this global filter was not created
as part of the composite filter. This caused a gap with "NULL"
value in the composite filter array which ended up with the rest
of the filters after the gap to be ignored and also it caused a mem
leak when destroying the composite filter.