Don't use un-initialized stride to base constructor

We can use a dummy value here as we set up the buffer and stride
further down in the constructor.
diff --git a/vncviewer/PlatformPixelBuffer.cxx b/vncviewer/PlatformPixelBuffer.cxx
index 1e9803e..ff1935e 100644
--- a/vncviewer/PlatformPixelBuffer.cxx
+++ b/vncviewer/PlatformPixelBuffer.cxx
@@ -36,7 +36,7 @@
 PlatformPixelBuffer::PlatformPixelBuffer(int width, int height) :
   FullFramePixelBuffer(rfb::PixelFormat(32, 24, false, true,
                                         255, 255, 255, 16, 8, 0),
-                       width, height, 0, stride),
+                       width, height, NULL, 0),
   Surface(width, height)
 #if !defined(WIN32) && !defined(__APPLE__)
   , shminfo(NULL), xim(NULL)