qemu: Validate arg in qemuAgentErrorComandUnsupported()
qemu: Validate arg in qemuAgentErrorComandUnsupported()

Coverity noted that 'reply' can be NULL after calling
qemuAgentCommand().  Avoid dereferencing reply in
qemuAgentErrorComandUnsupported() in that case.

Signed-off-by: Jonathon Jongsma <c7254805a17bd5c41e24c3a278fcbf3afcdfb0fa@redhat.com>

fabianfreyer/libvirt,crobinso/libvirt,andreabolognani/libvirt,zippy2/libvirt,olafhering/libvirt,jardasgit/libvirt,andreabolognani/libvirt,libvirt/libvirt,olafhering/libvirt,jfehlig/libvirt,jfehlig/libvirt,jardasgit/libvirt,nertpinx/libvirt,crobinso/libvirt,crobinso/libvirt,nertpinx/libvirt,zippy2/libvirt,crobinso/libvirt,zippy2/libvirt,jfehlig/libvirt,jfehlig/libvirt,olafhering/libvirt,libvirt/libvirt,jardasgit/libvirt,jardasgit/libvirt,libvirt/libvirt,andreabolognani/libvirt,andreabolognani/libvirt,fabianfreyer/libvirt,andreabolognani/libvirt,nertpinx/libvirt,olafhering/libvirt,zippy2/libvirt,fabianfreyer/libvirt,jardasgit/libvirt,fabianfreyer/libvirt,nertpinx/libvirt,nertpinx/libvirt,fabianfreyer/libvirt,libvirt/libvirt