eof: Cleanup stack height validation for next instruction

[chfast]

The stack height check for the next instruction is always successful. Downgrade the check of the result of the visit_successor in this case to assert.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.50%. Comparing base (0be7b16) to head (960bb5f).
Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1144   +/-   ##
=======================================
  Coverage   94.50%   94.50%           
=======================================
  Files         167      167           
  Lines       18193    18193           
=======================================
+ Hits        17193    17194    +1     
+ Misses       1000      999    -1     

Flag	Coverage Δ
eof_execution_spec_tests	15.34% <100.00%> (+<0.01%)	⬆️
ethereum_tests	28.32% <100.00%> (+<0.01%)	⬆️
ethereum_tests_silkpre	20.83% <0.00%> (ø)
execution_spec_tests	21.10% <0.00%> (ø)
unittests	89.72% <100.00%> (+<0.01%)	⬆️

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Files with missing lines	Coverage Δ
lib/evmone/eof.cpp	86.06% <100.00%> (+0.17%)	⬆️