Compilers that don't realize that ereport(ERROR) doesn't return
complained that XmlTableGetValue() failed to return a value.

Also, make XmlTableFetchRow's non-USE_LIBXML case look more like
the other ones.  As coded, it could lead to "unreachable code"
warnings with USE_LIBXML enabled.

Oversights in commit fcec6caa.  Per buildfarm.