parse_expr.c:typecast_expression:return immediate on NULL input

in parse_expr.c:typecast_expression: return immediate on NULL input,
i.e. do not lookup target type Oid as it is not required.

-Hannes
Index: src/backend/parser/parse_expr.c
===================================================================
--- src/backend/parser/parse_expr.c    (revision 377)
+++ src/backend/parser/parse_expr.c    (working copy)
@@ -2226,11 +2226,11 @@
     Oid            targetType;
     int32        targetTypmod;

-    targetType = typenameTypeId(pstate, typename, &targetTypmod);
-
     if (inputType == InvalidOid)
         return expr;            /* do nothing if NULL input */

+    targetType = typenameTypeId(pstate, typename, &targetTypmod);
+
     expr = coerce_to_target_type(pstate, expr, inputType,
                                  targetType, targetTypmod,
                                  COERCION_EXPLICIT,