Defensively fix 3 asserts that didn't hold and prevent endless loop in md_rollback

[TCKnet]

These were discovered through fuzzying

[codecov[bot]]

Codecov Report

Merging #182 (2ac0b9d) into master (e9ff661) will decrease coverage by 0.08%. The diff coverage is 70.00%.

:exclamation: Current head 2ac0b9d differs from pull request most recent head dbb7889. Consider uploading reports for the commit dbb7889 to get more accurate results

@@            Coverage Diff             @@
##           master     #182      +/-   ##
==========================================
- Coverage   94.32%   94.23%   -0.09%     
==========================================
  Files           3        3              
  Lines        3100     3105       +5     
==========================================
+ Hits         2924     2926       +2     
- Misses        176      179       +3     

Impacted Files	Coverage Δ
src/md4c.c	94.13% <70.00%> (-0.10%)	:arrow_down:

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[TCKnet]

Test cases: testcases.zip