output: avoid crashing on no crtc

dual gpus can init with no crtc and later rescan for them, however
getRenderFormats might be called before that happends and null ptr
deref, guard against it.

src/backend/drm/DRM.cpp

@@ -1713,6 +1713,10 @@ size_t Aquamarine::CDRMOutput::getGammaSize() {
 }
 
 std::vector<SDRMFormat> Aquamarine::CDRMOutput::getRenderFormats() {
+    if (!connector->crtc || !connector->crtc->primary || connector->crtc->primary->formats.empty()) {
+        backend->log(AQ_LOG_ERROR, "Can't get formats: no crtc");
+        return {};
+    }
     return connector->crtc->primary->formats;
 }