Fix test-commodities so it doesn't crash anymore.

[Derek Atkins]

Log Message:
-----------
Fix test-commodities so it doesn't crash anymore.

	* src/engine/gnc-commodity.[ch]:
	  - Use gnc_commodity_table_find_namespace() throughout
	  - fix a crash when adding a new namespace to the commodity table.

Tags:
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gnucash-gnome2-dev

Modified Files:
--------------
    gnucash:
        ChangeLog
    gnucash/src/engine:
        gnc-commodity.c
        gnc-commodity.h