virsh: don't call virSecretFree on NULL
virsh: don't call virSecretFree on NULL

Since the refactoring in fbe2d49 we call virSecretFree even if
virSecretDefineXML fails, which leads to overwriting the error
message with:
error: Invalid secret: virSecretFree

Bug: https://bugzilla.redhat.com/show_bug.cgi?id=929045

andreabolognani/libvirt,olafhering/libvirt,jardasgit/libvirt,VenkatDatta/libvirt,nertpinx/libvirt,rlaager/libvirt,crobinso/libvirt,agx/libvirt,andreabolognani/libvirt,taget/libvirt,zhlcindy/libvirt-1.1.4-maintain,agx/libvirt,olafhering/libvirt,rlaager/libvirt,cbosdo/libvirt,fabianfreyer/libvirt,shugaoye/libvirt,eskultety/libvirt,nertpinx/libvirt,crobinso/libvirt,zippy2/libvirt,shugaoye/libvirt,crobinso/libvirt,jardasgit/libvirt,datto/libvirt,olafhering/libvirt,jfehlig/libvirt,VenkatDatta/libvirt,VenkatDatta/libvirt,elmarco/libvirt,zhlcindy/libvirt-1.1.4-maintain,shugaoye/libvirt,zippy2/libvirt,jardasgit/libvirt,elmarco/libvirt,eskultety/libvirt,jfehlig/libvirt,crobinso/libvirt,taget/libvirt,VenkatDatta/libvirt,libvirt/libvirt,shugaoye/libvirt,agx/libvirt,andreabolognani/libvirt,agx/libvirt,taget/libvirt,fabianfreyer/libvirt,jfehlig/libvirt,fabianfreyer/libvirt,jardasgit/libvirt,jardasgit/libvirt,cbosdo/libvirt,fabianfreyer/libvirt,datto/libvirt,zhlcindy/libvirt-1.1.4-maintain,datto/libvirt,libvirt/libvirt,andreabolognani/libvirt,elmarco/libvirt,zhlcindy/libvirt-1.1.4-maintain,cbosdo/libvirt,fabianfreyer/libvirt,shugaoye/libvirt,zippy2/libvirt,eskultety/libvirt,zippy2/libvirt,rlaager/libvirt,taget/libvirt,rlaager/libvirt,cbosdo/libvirt,elmarco/libvirt,nertpinx/libvirt,libvirt/libvirt,cbosdo/libvirt,zhlcindy/libvirt-1.1.4-maintain,datto/libvirt,jfehlig/libvirt,rlaager/libvirt,libvirt/libvirt,nertpinx/libvirt,eskultety/libvirt,nertpinx/libvirt,VenkatDatta/libvirt,olafhering/libvirt,elmarco/libvirt,datto/libvirt,andreabolognani/libvirt,eskultety/libvirt,agx/libvirt,taget/libvirt