filter-apply: Ignore monitor source of filter in find_paired_master()

filter-apply: Ignore monitor source of filter in find_paired_master()

When module-filter-apply tries to find a matching source-output for
a given sink-input and a stream within the same group exists on the
monitor source of the filter, module-filter apply falsely assumes
that the source belongs to another instance of the filter and tries
to access source->output_from_master->source, which leads to a
segmentation fault.

This patch fixes the issue by ignoring the stream if the source is
the monitor source of the filter.