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: NAshish Mittal <ashmit602@gmail.com>

src/qemu/qemu_hotplug.c

@@ -1638,12 +1638,14 @@ qemuDomainGetTLSObjects(virQEMUCapsPtr qemuCaps,
         if (qemuBuildSecretInfoProps(secinfo, secProps) < 0)
             return -1;
 
-        if (!(*secAlias = qemuDomainGetSecretAESAlias(srcAlias, false)))
+        if (!secAlias ||
+            !(*secAlias = qemuDomainGetSecretAESAlias(srcAlias, false)))
             return -1;
     }
 
     if (qemuBuildTLSx509BackendProps(tlsCertdir, tlsListen, tlsVerify,
-                                     *secAlias, qemuCaps, tlsProps) < 0)
+                                     secAlias ? *secAlias : NULL, qemuCaps,
+                                     tlsProps) < 0)
         return -1;
 
     if (!(*tlsAlias = qemuAliasTLSObjFromSrcAlias(srcAlias)))