tests: vhost-user-test: release mutex on protocol violation

chr_read() is printing an error message and returning with s->data_mutex taken.
This can potentially cause a hang.  Reported by Coverity.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

tests/qtest/vhost-user-test.c

@@ -351,7 +351,7 @@ static void chr_read(void *opaque, const uint8_t *buf, int size)
         if (size != msg.size) {
             qos_printf("%s: Wrong message size received %d != %d\n",
                        __func__, size, msg.size);
-            return;
+            goto out;
         }
     }
 
@@ -509,6 +509,7 @@ static void chr_read(void *opaque, const uint8_t *buf, int size)
         break;
     }
 
+out:
     g_mutex_unlock(&s->data_mutex);
 }