qemu: Avoid a possible NULL pointer deref in qemuDomainGetTLSObjects
Passing a NULL value for the argument secAlias to the function
qemuDomainGetTLSObjects would cause a segmentation fault in
libvirtd.
Changed code to check before dereferencing a NULL secAlias.
Signed-off-by: Ashish Mittal <ashmit602@gmail.com>