user32: implement return value for DdeClientTransaction for XTYP_EXECUTE

[Dan Kegel]

This fixes a couple todo_wine's.
It'd be nice if a dde expert reviewed this, since I
don't know jack about dde, but it seems reasonable.

Found via valgrind warning
 Conditional jump or move depends on uninitialised value(s)
    at 0x46BB612: winetest_ok (test.h:261)
    by 0x4601484: func_dde (dde.c:406)
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