opusenc: fix segmentation fault at x86 version

The argument 0x0 is interpreted by the x86 compiler as a 32-bit int, but
it is consumed as a 64-bit uint causing a segmentation fault. We need to
explicit cast it to guint64 in order for the va_list to be built correctly.

https://bugzilla.gnome.org/show_bug.cgi?id=797092
1 file changed