userpref: [GnuTLS] Use valid serial for >= 3.6.0
userpref: [GnuTLS] Use valid serial for >= 3.6.0

Another change in 3.6.0 is that a serial of '\0' is not valid anymore.
Bump it to one.

libimobiledevice-win32/libimobiledevice,Tatsh/libimobiledevice,libimobiledevice-win32/libimobiledevice,libimobiledevice-win32/libimobiledevice,libimobiledevice-win32/libimobiledevice,Tatsh/libimobiledevice,Tatsh/libimobiledevice,libimobiledevice/libimobiledevice,libimobiledevice/libimobiledevice,Tatsh/libimobiledevice