BUG #5757: PL/PGSQL composite variable field error reporting

The following bug has been logged online:

Bug reference:      5757
Logged by:          Andrey Galkin
Email address:      andvgal@gmail.com
PostgreSQL version: 9.0.1
Operating system:   Debian unstable
Description:        PL/PGSQL composite variable field error reporting
Details:

1. Create a variable of composite type in PL/PGSQL function
2. Try to reference not existing field of the variable in some statement,
like IF
3. Get "missing FROM-clause entry for table" error at runtime

The error is not very helpful. If it's possible please detect the
incorrectly referenced field at parsing/statement preparing time.