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

tools/virsh-secret.c

@@ -117,7 +117,8 @@ cmdSecretDefine(vshControl *ctl, const vshCmd *cmd)
 
 cleanup:
     VIR_FREE(buffer);
-    virSecretFree(res);
+    if (res)
+        virSecretFree(res);
     return ret;
 }