Fix applicability condition of simplify_byte_update

[peterschrammel]

The condition is supposed to guard against a crash in basic_string::replace in line 1896, which it fails to do if the offsets in the condition and the argument to replace don't match.

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[codecov]

Codecov Report

Merging #6311 (bbdfdb1) into develop (493bc71) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff            @@
##           develop    #6311   +/-   ##
========================================
  Coverage    75.98%   75.98%           
========================================
  Files         1512     1512           
  Lines       163456   163456           
========================================
  Hits        124204   124204           
  Misses       39252    39252           

Impacted Files	Coverage Δ
src/util/simplify_expr.cpp	84.71% <100.00%> (ø)

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[TGWDB]

Looks ok, but it would be nice if we didn't have a magic number.

[peterschrammel]

Looks ok, but it would be nice if we didn't have a magic number.

This will be done in #917