fixes entry for PR #884 - X11 extended command

Pull request from entrez:  X11's get-extended-command widget
allocated 1 extra byte for a couple of end-or-array terminators
and then wrote more than 1 byte to them when initializing its list
of extended commands.

Fixes #884

doc/fixes3-7-0.txt

@@ -1647,6 +1647,8 @@ X11: (possibly X11+OSX): if persistent inventory was displayed at time of
 	end-of-game prompting, prompting would stall until that window was
 	manually dismissed
 X11: set all selectable menu lines to the same length, left justified
+X11: initializing the get-extended-command widget modified memory beyond what
+	it dynamically allocated
 
 
 General New Features