Chris Shoemaker's patch: Avoid use of unitialized values in guid.c.

[Derek Atkins]

Log Message:
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Chris Shoemaker's patch: Avoid use of unitialized values in guid.c.

	* Avoid use of uninitialized values in guid.c
	  - prevent md5 seeding from using uninitialized stack contents
	  - give used/null GUID recognizable memory signature
	  - add a simple test case that helped testing for use of
	    uninitialized values

Tags:
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gnucash-gnome2-dev

Modified Files:
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    gnucash:
        ChangeLog
    gnucash/src/engine:
        guid.c
    gnucash/src/engine/test:
        test-guid.c