physicalplan: fix minor leak with pooling of FlowSpecs

The only thing that we reuse when pooling `FlowSpec` objects is the
capacity of `Processors` slice. Previously, we forgot to unset each
element of that slice, so we could have hold onto to some processor
specs which in turn could have hold onto some large objects (like
`roachpb.Span`s in the TableReader) until a particular index of the
slice is overwritten. This oversight is now fixed.

Release note: None

Author: yuzefovich

pkg/sql/physicalplan/specs.go

@@ -34,6 +34,7 @@ func ReleaseFlowSpec(spec *execinfrapb.FlowSpec) {
 		if tr := spec.Processors[i].Core.TableReader; tr != nil {
 			releaseTableReaderSpec(tr)
 		}
+		spec.Processors[i] = execinfrapb.ProcessorSpec{}
 	}
 	*spec = execinfrapb.FlowSpec{
 		Processors: spec.Processors[:0],