Reloadable shotgun: fix primary fire triggering melee attack too if clip has some...

Reloadable shotgun: fix primary fire triggering melee attack too if clip has some ammo but player has none

Implementation notes:
- the first return was added to fix only the case where clip has exactly 1 bullet but player has none
- the second return was added for consistency's sake and avoids wasting a couple checks because if(WEP_CVAR(shotgun, secondary) == 1) will always return false