Fix error handling path in SRFs

It should be safe to unregister a context callback even when in an
aborted subxact, the only possible errors are memory ones.

Fixes pllua#15.

expected/spi.out

@@ -260,4 +260,11 @@ fetch all from mycur2;
 (6 rows)
 
 commit;
+-- check correct error handling inside coroutines (issue #15)
+create function pg_temp.f1() returns setof text language pllua as $$
+  local r = spi.rows('syntaxerror')
+  return 'notreached'
+$$;
+select * from pg_temp.f1();
+ERROR:  syntax error at or near "syntaxerror" at character 1
 --end

sql/spi.sql

@@ -202,4 +202,12 @@ do language pllua $$ collectgarbage() $$;  -- check cursor stays open
 fetch all from mycur2;
 commit;
 
+-- check correct error handling inside coroutines (issue #15)
+
+create function pg_temp.f1() returns setof text language pllua as $$
+  local r = spi.rows('syntaxerror')
+  return 'notreached'
+$$;
+select * from pg_temp.f1();
+
 --end

src/objects.c

@@ -660,7 +660,13 @@ void pllua_deactivate_thread(lua_State *L, pllua_func_activation *act, ExprConte
 {
 	Assert(act->thread != NULL);
 
-	PLLUA_TRY();
+	/*
+	 * We need to be able to do this on error paths, and the unregister call
+	 * should only ever error on logic or memory corruption cases and can't
+	 * reach any user-supplied code, so it should be safe to use the _ERROK
+	 * variant.
+	 */
+	PLLUA_TRY_ERROK();
 	{
 		UnregisterExprContextCallback(econtext,
 									  pllua_resetactivation_cb,