qemu: sound: Fix uninitialized model string

Commit e5f36698 introduces a
false-positive build failure in the sound card model handling switch.
Initialize the model to NULL although the value should never be used.

src/qemu/qemu_command.c

@@ -4674,7 +4674,7 @@ qemuBuildSoundDevStr(virDomainDefPtr def,
                      virQEMUCapsPtr qemuCaps)
 {
     virBuffer buf = VIR_BUFFER_INITIALIZER;
-    const char *model;
+    const char *model = NULL;
 
     /* Hack for devices with different names in QEMU and libvirt */
     switch ((virDomainSoundModel) sound->model) {