physicalplan: harden PhysicalInfrastructure.Release

We sync-pool `PhysicalInfrastructure` objects and previously we wouldn't
explicitly unset elements of `Processors` slice. This was done since
it's a slice of values not pointers. However, those values themselves
embed protobuf ProcessorSpec which contains more messages (among other
things might have RenderExprs) which we do want to lose the references
to, so this commit fixes that oversight.

Release note: None

Author: yuzefovich

pkg/sql/physicalplan/physical_plan.go

@@ -193,8 +193,9 @@ func NewPhysicalInfrastructure(
 
 // Release resets the object and puts it back into the pool for reuse.
 func (p *PhysicalInfrastructure) Release() {
-	// We only need to nil out the local processors since these are the only
-	// pointer types.
+	for i := range p.Processors {
+		p.Processors[i] = Processor{}
+	}
 	for i := range p.LocalProcessors {
 		p.LocalProcessors[i] = nil
 	}