Bugfix With Mechs (mecha.dm)

Added in a check to see if the mech a player is trying to enter has a health value greater than 0. This fixes a bug whereby if a player is entering a mech as it is destroyed, their mob gets deleted and the player loses connection to the server. Reconnecting to the server allows this player to rejoin the game, but as a new person.
Merge pull request #4329 from Justin1693/patch-1

Bugfix For Mechs