Fix ListPoster crash

[cewert]

Prevent crash by validating we have access to global constants before using. Comes from roku.com crash log

Crashlog:

posterbackgrounds <uninitialized> 
m                roAssociativeArray refcnt=2 count:11 
global           Interface:ifGloba$1 Local Variables: 
   file/line: pkg:/components/ListPoster.brs(17) 
#0  Function init() As Voi$1 Backtrace: 
'Dot' Operator attempted with invalid BrightScript Component or interface reference. (runtime error &hec) in pkg:/components/ListPoster.brs(17)

which points to this line after running build-prod on 2.1.1:

posterBackgrounds = m.global.constants.poster_bg_pallet

Issues

Ref #1164