pgsql: Tweak newly added set_config_sourcefile() so that the target

Log Message:
-----------
Tweak newly added set_config_sourcefile() so that the target record
isn't left corrupt if guc_strdup should fail.

Modified Files:
--------------
pgsql/src/backend/utils/misc:
guc.c (r1.471 -> r1.472)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/misc/guc.c?r1=1.471&r2=1.472)