eth/filters: terminate pending tx subscription on error (#32794)

Fixes issue #32793. When the pending tx subscription ends, the filter
is removed from `api.filters`, but it is not terminated. There is no other 
way to terminate it, so the subscription will leak, and potentially block
the producer side.

eth/filters/api.go

@@ -152,6 +152,7 @@ func (api *FilterAPI) NewPendingTransactionFilter(fullTx *bool) rpc.ID {
 				api.filtersMu.Lock()
 				delete(api.filters, pendingTxSub.ID)
 				api.filtersMu.Unlock()
+				pendingTxSub.Unsubscribe()
 				return
 			}
 		}