#3288 null overrides crashes (#3289)

* Fixes for #3288 override copy related crashes when overrides can be nullptr

Author: brad-linden

indra/newview/llselectmgr.cpp

@@ -2251,11 +2251,15 @@ void LLSelectMgr::selectionRevertGLTFMaterials()
 
                 // Update material locally
                 objectp->setRenderMaterialID(te, asset_id, false /*wait for LLGLTFMaterialList update*/);
-                LLGLTFMaterial* material = new LLGLTFMaterial(*nodep->mSavedGLTFOverrideMaterials[te]);
-                objectp->setTEGLTFMaterialOverride(te, material);
+                LLGLTFMaterial* material = nodep->mSavedGLTFOverrideMaterials[te];
+                if (material)
+                {
+                    material = new LLGLTFMaterial(*material);
+                    objectp->setTEGLTFMaterialOverride(te, material);
+                }
 
                 // Enqueue update to server
-                if (asset_id.notNull())
+                if (asset_id.notNull() && material)
                 {
                     // Restore overrides and base material
                     LLGLTFMaterialList::queueApply(objectp, te, asset_id, material);

indra/newview/lltooldraganddrop.cpp

@@ -1447,9 +1447,10 @@ void LLToolDragAndDrop::dropTexture(LLViewerObject* hit_obj,
             }
 
             LLTextureEntry* te = hit_obj->getTE(hit_face);
-            if (te && !remove_pbr)
+            LLGLTFMaterial * override_mat = nullptr;
+            if (te && !remove_pbr && (override_mat = te->getGLTFMaterialOverride()))
             {
-                LLGLTFMaterial* copy = new LLGLTFMaterial(*te->getGLTFMaterialOverride());
+                LLGLTFMaterial* copy = new LLGLTFMaterial(*override_mat);
                 nodep->mSavedGLTFOverrideMaterials[hit_face] = copy;
             }
             else