qemu: Fix crash in virQEMUCapsProbeQMPTPM
qemu: Fix crash in virQEMUCapsProbeQMPTPM

If the queried QMP command doesn't exist qemuMonitorGetTPMModels returns
0 but sets the string list to NULL which isn't accepted by
g_strv_contains.

Fixes: a5bc5f0ecf8
Reported-by: Olaf Hering <ded022db89399a060a39da31922f173c6556c2d9@aepfle.de>
Signed-off-by: Peter Krempa <2cf5c04c61aa466e4a47bfedc747d17279c72ffc@redhat.com>
Reviewed-by: Erik Skultety <2c14d38fa47c8799f1b9c16280abe27f8edfec6e@redhat.com>

libvirt/libvirt,crobinso/libvirt,olafhering/libvirt,zippy2/libvirt,libvirt/libvirt,libvirt/libvirt,nertpinx/libvirt,jfehlig/libvirt,jfehlig/libvirt,nertpinx/libvirt,nertpinx/libvirt,zippy2/libvirt,olafhering/libvirt,jfehlig/libvirt,jfehlig/libvirt,crobinso/libvirt,olafhering/libvirt,crobinso/libvirt,zippy2/libvirt,zippy2/libvirt,nertpinx/libvirt,nertpinx/libvirt,libvirt/libvirt,olafhering/libvirt,crobinso/libvirt