pgsql: The recent patch to log changes in postgresql.conf settings

Log Message:
-----------
The recent patch to log changes in postgresql.conf settings dumped core
if the initial value of a string variable was NULL, which is entirely
possible.  Noted while experimenting with custom_variable_classes.

Modified Files:
--------------
    pgsql/src/backend/utils/misc:
        guc-file.l (r1.62 -> r1.63)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/misc/guc-file.l?r1=1.62&r2=1.63)