MG crashes when creation time of an unit is zero

[MirceaKitsune]

If the creation time of an unit is set to zero (<time value="0" />) Megaglest will crash with a "floating point exception" error when the AI (CPU Mega) attempts to build it, although nothing bad seems to happen if the player builds it in a local match. Can be solved by setting it to a higher value, but I don't think it's normal for it to actually crash the engine so I still thought to report.