[pitivi] previewers: Fix usage of field not yet set

[Alexandru Băluț <alexbalut src gnome org>]

commit 7cc8df315d850ce4f1544b74775c79c06f0c0d30
Author: Alexandru Băluț <alexandru balut gmail com>
Date:   Sun Sep 15 00:27:45 2019 +0200

    previewers: Fix usage of field not yet set
    
    For some reason WaveformPreviewer.do_post_message is now triggered
    before the constructor finishes:
    
    >   File "/root/pitivi/pitivi/timeline/previewers.py", line 214, in do_post_message
    >     if not self.passthrough and \
    > 'WaveformPreviewer' object has no attribute 'passthrough'

 pitivi/timeline/previewers.py | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)
---
diff --git a/pitivi/timeline/previewers.py b/pitivi/timeline/previewers.py
index ed57e0cb..6170380e 100644
--- a/pitivi/timeline/previewers.py
+++ b/pitivi/timeline/previewers.py
@@ -212,9 +212,9 @@ class WaveformPreviewer(PreviewerBin):
 
     # pylint: disable=arguments-differ
     def do_post_message(self, message):
-        if not self.passthrough and \
-                message.type == Gst.MessageType.ELEMENT and \
-                message.src == self.level:
+        if message.type == Gst.MessageType.ELEMENT and \
+                message.src == self.level and \
+                not self.passthrough:
             struct = message.get_structure()
             peaks = None
             if struct: