[PATCH] Structure Tree - Aristeia's egyptian phase-pair technologies cause errors

[s0600204]

To reproduce:

• Run 0ad with the aristeia mod enabled
• Go to the structure tree (either off the main menu, or by clicking on the emblem top-centre in-game)
• Choose the 'Egyptian' civ from the dropdown
• Notice the error message

The error is caused because the mod's Egyptian civ presents a choice of two possible city-phase techs to the player, and the structree is unable to cope with this.

The attached patch fixes:

• The structree's inability to deal with a phase-pair
• The fact that units dependant on said phase-pair were not appearing where they should or, in fact, at all

[Itms]

In 17073:

Make the structure tree support paired technologies. Patch by s0600204, fixes #3401.

[Itms]

I just realized my commit message is not really accurate. Anyway, I enhanced the style a bit (trailing space, more curly braces, and early returns).

Thanks for this patch :)