qtdemux: Fix wrong compiler warning with gcc 6.2
qtdemux: Fix wrong compiler warning with gcc 6.2

| ../../../git/gst/isomp4/qtdemux.c: In function 'qtdemux_parse_tree':
| ../../../git/gst/isomp4/qtdemux.c:10224:24: error: 'size' may be used uninitialized in this function [-Werror=maybe-uninitialized]
|                  offset += size;
|                         ^~
| ../../../git/gst/isomp4/qtdemux.c:10197:25: note: 'size' was declared here
|                  guint32 size, tag;
|                          ^~~~

https://bugzilla.gnome.org/show_bug.cgi?id=774747

GStreamer/gst-plugins-good,GStreamer/gst-plugins-good,GrokImageCompression/gst-plugins-good,GStreamer/gst-plugins-good,StreamUtils/gst-plugins-good,StreamUtils/gst-plugins-good,StreamUtils/gst-plugins-good,stfl/gst-plugins-good,GStreamer/gst-plugins-good,hizukiayaka/gst-plugins-good,stfl/gst-plugins-good,GrokImageCompression/gst-plugins-good,hizukiayaka/gst-plugins-good,stfl/gst-plugins-good,StreamUtils/gst-plugins-good,hizukiayaka/gst-plugins-good,pexip/gst-plugins-good,hizukiayaka/gst-plugins-good,GrokImageCompression/gst-plugins-good,pexip/gst-plugins-good,pexip/gst-plugins-good,GrokImageCompression/gst-plugins-good,stfl/gst-plugins-good,pexip/gst-plugins-good,pexip/gst-plugins-good