COPY: Add an assertion

This is for tools such as Coverity that don't know that the grammar
enforces that the case of not having a relation (but instead a query)
cannot happen in the FROM case.

src/backend/commands/copy.c

@@ -797,6 +797,8 @@ DoCopy(const CopyStmt *stmt, const char *queryString)
 
 	if (is_from)
 	{
+		Assert(rel);
+
 		/* check read-only transaction */
 		if (XactReadOnly && rel->rd_backend != MyBackendId)
 			PreventCommandIfReadOnly("COPY FROM");