[LyX/master] Fix typo

[Georg Baum]

commit 3602baa40c283f32fea3dcfde08b12306afe3dab
Author: Georg Baum <b...@lyx.org>
Date:   Fri Oct 9 21:37:43 2015 +0200

    Fix typo
    
    Found by gcc: operation on 'preview_ptr' may be undefined [-Wsequence-point]

diff --git a/src/insets/InsetExternal.cpp b/src/insets/InsetExternal.cpp
index a62100e..cb86df4 100644
--- a/src/insets/InsetExternal.cpp
+++ b/src/insets/InsetExternal.cpp
@@ -617,7 +617,7 @@ void InsetExternal::setParams(InsetExternalParams const & p)
                break;
        case PREVIEW_INSTANT: {
                renderer_.reset(new RenderMonitoredPreview(this));
-               RenderMonitoredPreview * preview_ptr = preview_ptr = 
renderer_->asMonitoredPreview();
+               RenderMonitoredPreview * preview_ptr = 
renderer_->asMonitoredPreview();
                preview_ptr->fileChanged(bind(&InsetExternal::fileChanged, 
this));
                if (preview_ptr->monitoring())
                        preview_ptr->stopMonitoring();