Fix Asan warning in property editor texture tab

When there is no active texture, a NULL pointer was dereferenced.
It didn't crash because the dereference was for the first item at the
pointer, the ID. To fix this, return with no data when `texture is NULL.

source/blender/editors/space_buttons/buttons_context.c

@@ -880,6 +880,10 @@ int /*eContextResult*/ buttons_context(const bContext *C,
     ButsContextTexture *ct = sbuts->texuser;
 
     if (ct) {
+      if (ct->texture == NULL) {
+        return CTX_RESULT_NO_DATA;
+      }
+
       CTX_data_pointer_set(result, &ct->texture->id, &RNA_Texture, ct->texture);
     }