Do not init stack space to zero

[chfast]

This proposes not to init stack space to zero. In theory, EVM should never read a stack item which has not been initialized previously with push or any other instruction. In practice, this change breaks at least Advanced. Problems are reported by valgrind and ASan e.g. in "dup_stack_overflow" test.

[codecov[bot]]

Codecov Report

Merging #426 (34eb6cb) into master (74377f6) will increase coverage by 0.00%. The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #426   +/-   ##
=======================================
  Coverage   99.62%   99.62%           
=======================================
  Files          31       31           
  Lines        4017     4020    +3     
=======================================
+ Hits         4002     4005    +3     
  Misses         15       15           

Flag	Coverage Δ
consensus	86.51% <100.00%> (ø)
unittests	99.67% <100.00%> (+<0.01%)	:arrow_up:

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
lib/evmone/execution_state.hpp	94.82% <100.00%> (+0.28%)	:arrow_up:
test/unittests/execution_state_test.cpp	100.00% <100.00%> (ø)