Serverside `gmod_wire_spu` NULL error

[wrefgtzweve]

Saw this error in the logs, seems like it might be relatively easy to fix. This happened on latest master.

addons/wire/lua/wire/gpulib.lua:749: Tried to use a NULL entity!
   1.  unknown - [C]:-1
    2.  Start - [C]:-1
     3.  Flush - addons/wire/lua/wire/gpulib.lua:749
      4.  unknown - addons/wire/lua/entities/gmod_wire_spu/init.lua:119

locals:

compressInfo = {
  1 = Table [6 items]
  2 = Table [6 items]
  3 = Table [6 items]
  4 = Table [6 items]
  5 = Table [5 items]
}
self         = {
  CacheBytes = Number [0]
  Entity = Entity [2880][gmod_wire_spu]
  EntIndex = Number [2880]
  Enabled = Bool [true]
  Cache = Table [1582 items]
}
forcePlayer  = Player [NULL]
messageSize  = Number [4]

[wrefgtzweve]

Should be fixed in #2580