Fix loop using wrong array's size

[Muhlex]

Seems like this loop was using the wrong ArrayList's size when iterating in initialization.

Caused this exception on startup for me:

L 09/18/2020 - 14:21:14: [navmesh.smx] Found .NAV file in maps\workshop/1685313095/mg_gradient.nav
L 09/18/2020 - 14:21:14: [navmesh.smx] Is mesh analyzed: 1
L 09/18/2020 - 14:21:14: [navmesh.smx] Nav version: 16; SubVersion: 1 (v10+); BSPSize: 5894896; MagicNumber: -17958194
L 09/18/2020 - 14:21:14: [navmesh.smx] Place count: 0
L 09/18/2020 - 14:21:14: [navmesh.smx] Has unnamed areas: true
L 09/18/2020 - 14:21:14: [navmesh.smx] Area count: 2840
L 09/18/2020 - 14:21:16: [navmesh.smx] Parsed 2840 areas in 2.852231 seconds.
L 09/18/2020 - 14:21:16: [navmesh.smx] Ladder count: 0
L 09/18/2020 - 14:21:16: [SM] Exception reported: Invalid index 38513 (count: 38513)
L 09/18/2020 - 14:21:16: [SM] Blaming: navmesh.smx
L 09/18/2020 - 14:21:16: [SM] Call stack trace:
L 09/18/2020 - 14:21:17: [SM]   [0] ArrayList.Get
L 09/18/2020 - 14:21:17: [SM]   [1] Line 1705, c:\srcds\csgo\addons\sourcemod\scripting\navmesh.sp::NavMeshLoad
L 09/18/2020 - 14:21:17: [SM]   [2] Line 396, c:\srcds\csgo\addons\sourcemod\scripting\navmesh.sp::OnMapStart

[KitRifty]

Very nice catch. Thanks!