lv2: only use g_return_val_if_fail for checking pre-conditions

g_return_* can be disabled. Still we want to check for failed instance creation.
diff --git a/ext/lv2/gstlv2.c b/ext/lv2/gstlv2.c
index 99e5f5f..067d1c0 100644
--- a/ext/lv2/gstlv2.c
+++ b/ext/lv2/gstlv2.c
@@ -612,9 +612,9 @@
 
   GST_DEBUG_OBJECT (lv2, "instantiating the plugin at %d Hz", sample_rate);
 
-  lv2->instance = slv2_plugin_instantiate (oclass->plugin, sample_rate, NULL);
-
-  g_return_val_if_fail (lv2->instance != NULL, FALSE);
+  if (!(lv2->instance =
+          slv2_plugin_instantiate (oclass->plugin, sample_rate, NULL)))
+    goto no_instance;
 
   /* connect the control ports */
   for (i = 0; i < gsp_class->num_control_in; i++)
@@ -633,6 +633,11 @@
     GST_WARNING_OBJECT (gsp, "got no sample-rate");
     return FALSE;
   }
+no_instance:
+  {
+    GST_WARNING_OBJECT (gsp, "could not create instance");
+    return FALSE;
+  }
 }
 
 static gboolean