render_weapon_hud set the secondary magazine's overlay flags into slots 0
and 1, over the primary's, where its states and numbers use 4 and 5: a
weapon of one magazine showed its ammunition overlays as the empty second
magazine's (coldsnap's flamethrower read NO FUEL with a full tank).
A weapon of one magazine also read a second it doesn't have:
weapon_build_weapon_interface_state fills only the weapon's own, into
unset stack memory, so the HUD's secondary flash, reload and fired-empty
warnings followed whatever was there. The state is now cleared first, and
a weapon of one magazine keeps its secondary states and overlays at rest.