Allow transferring energy from shields to work properly at full shields.

A conditional originally added to shield_add_strength() to make shields not recharge beyond the recharge limit forgot to check that the amount shields are changing by was positive (unlike a couple other conditionals added/modified around the same time), so a call to shield_add_strength() with a negative delta (e.g. when transferring energy from shields to weapons) when the current shield strength is at or above the recharge limit (i.e. full strength when a recharge limit isn't specified) would immediately return without doing anything. This would, of course, result in no change to shield strength... so if you had full shields and had drained your weapon batteries dry, you could get free energy by hitting the button to transfer energy from your shields.

Author: MageKing17

code/object/objectshield.cpp

@@ -87,7 +87,7 @@ void shield_add_strength(object *objp, float delta) {
 	float shield_str = shield_get_strength(objp);
 	float shield_recharge_limit = shield_get_max_strength(objp);
 
-	if (shield_str >= shield_recharge_limit)
+	if ((delta > 0.0f) && (shield_str >= shield_recharge_limit))
 		return;
 
 	if (!(Ai_info[Ships[objp->instance].ai_index].ai_profile_flags[AI::Profile_Flags::Smart_shield_management])