Ensure that curSchema is set before opening the DB

Without this, it's possible to get `curSchema = 0` which then causes us
not to trigger the branch that maintains forward compatibility with the
1.12 schema.

Fixes #1332

src/libstore/local-store.cc

@@ -225,6 +225,7 @@ LocalStore::LocalStore(bool reserveSpace)
     schemaPath = settings.nixDBPath + "/schema";
 
     if (settings.readOnlyMode) {
+        curSchema = getSchema();
         openDB(false);
         return;
     }
@@ -309,6 +310,7 @@ LocalStore::LocalStore(bool reserveSpace)
     } catch (SysError & e) {
         if (e.errNo != EACCES) throw;
         settings.readOnlyMode = true;
+        curSchema = getSchema();
         openDB(false);
         return;
     }