sched/simulator: Add missing sim_state = NULL assignments

[lipari]

Corrects a problem that was revealed with
#238

[dongahn]

LGTM. I will merge this when Travis turns green.

[coveralls]

Coverage increased (+0.5%) to 73.434% when pulling c186435 on lipari:sim_state_null into ca349c9 on flux-framework:master.

[codecov-io]

Codecov Report

Merging #240 into master will increase coverage by 0.49%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master    #240      +/-   ##
=========================================
+ Coverage    55.2%   55.7%   +0.49%     
=========================================
  Files          25      25              
  Lines        5184    5186       +2     
  Branches     1179    1179              
=========================================
+ Hits         2862    2889      +27     
+ Misses       1615    1588      -27     
- Partials      707     709       +2

Impacted Files	Coverage Δ
sched/sched.c	57.14% <100%> (+1.03%)	⬆️
simulator/sim_execsrv.c	70.05% <100%> (+4.74%)	⬆️
simulator/submitsrv.c	69.93% <0%> (+1.22%)	⬆️
simulator/simsrv.c	54.18% <0%> (+2.95%)	⬆️

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