Tower shoots wrong number of arrows capturing

[elexis]

As of r17935 capturing towers leads to a wrong number of arrows being shot by the tower.

Reproduce:

1. Build a tower, garrison with 5 units -> shoots 5+1 arrows
2. Capture that tower with an enemy -> tooltip says '-4` arrows
3. Garrison tower with 5 units -> tooltip says 1 arrow

Things that might have broken it: r17882 r17783 r17863 or one/some of the other recent aura/auramanager changes.

Reverting r17931 didn't change the issue.

[sanderd17]

In 17938:

Fix counting of arrows in buildings: don't remove entities twice from the garrisonholder. Fixes #3846

[elexis]

In 17944:

Return Infinity instead of undefined in BuildingAI.GetMaxArrowCount. Refs #3196, #3846 and r17939.

Notice the previous check if (count > this.GetMaxArrowCount()) implicitly relied on
numerical comparisons returning false if one of the two arguments is undefined.