MCP altitude set to 0 when issuing dvs

[scott-j-smith]

Arriving aircraft given the command to "DVS" get an assigned altitude of 0 in the flight strip. See BCY858.

@Fechulo :
Arriving aircraft descend via the STAR by default, so there's no need to give them the 'dvs' command. >However, the assigned altitude shouldn't be 0, so that's something to look into.

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Steps to rectify:

1. The aircraft are reaching 0 feet, but are not considered to have crashed. Resolve whatever is causing this, and get them to crash.
2. Flight strip shows NaN upon the aircraft reaching 0 feet altitude. It should at least be showing 0 instead of NaN.
3. Despite the fact that the aircraft have had their autopilots set to 0 feet, this should never be allowed except during landing. The catch-all keeping them at least 1000ft above the airport elevation should still be in effect here.
4. The dvs command should not be setting the MCP altitude to 0, but rather the bottom altitude of the procedure (if there is one). If there is not one, we may want to consider setting the MCP altitude to the airspace ceiling if it is higher (to force a descent so we can control the aircraft on STARs that don't have built in crossing restrictions).