If there is a collate clause in the ORDER BY, the server crashes with assertion
+ Assert(loc_cxt.state == FDW_COLLATE_NONE ||
+ loc_cxt.state == FDW_COLLATE_SAFE);
The assertion is fine as long as is_foreign_expr() tests only boolean expressions (appearing in quals). This patch uses the function to test an expression appearing in ORDER BY clause, which need not be boolean. Attached patch removed the assertion and instead makes the function return false, when the walker deems collation of the expression unsafe. The walker can not return false when it encounter unsafe expression since the subtree it's examining might be part of an expression which does not use the collation ultimately.
