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: NJonathon Jongsma <jjongsma@redhat.com>

src/qemu/qemu_agent.c

@@ -1005,7 +1005,12 @@ static bool
 qemuAgentErrorCommandUnsupported(virJSONValuePtr reply)
 {
     const char *klass;
-    virJSONValuePtr error = virJSONValueObjectGet(reply, "error");
+    virJSONValuePtr error;
+
+    if (!reply)
+        return false;
+
+    error = virJSONValueObjectGet(reply, "error");
 
     if (!error)
         return false;