ensure that even for slices with no features, the adaptor bind params are left...

ensure that even for slices with no features, the adaptor bind params are left clean and empty once the method completes

modules/Bio/EnsEMBL/DBSQL/BaseFeatureAdaptor.pm

@@ -899,6 +899,7 @@ COORD_SYSTEM: foreach my $coord_system (@feature_coord_systems) {
      }
      $mapper = undef;
     } # End foreach
+    $self->{_bind_param_generic_fetch} = undef;
     return \@pan_coord_features;
 }
 #